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In the Harry Potter series, Expelliarmus is shown in different ways.

Sometimes, only a person's wand flies out of their hand, but sometimes the whole person flies across the room.

Example:

When Snape and Lockhart are duelling in CoS, Lockhart flies across the room.

Both of them swung their wands up over their shoulders. Snape cried: 'Expelliarmus!' There was a dazzling flash of scarlet light and Lockhart was blasted off his feet: he flew backwards off the stage, smashed into the wall and slid down it to sprawl on the floor.

Harry Potter and the Chamber of Secrets - p.142 - Bloomsbury - Chapter 11, The Duelling Club

But in The Prisoner of Azkaban, when Lupin cast Expelliarmus on Harry and Hermione in the Shrieking Shack, only the wands fly to Lupin.

Is it ever mentioned why there are different effects?

Is there any evidence in the books as to why this happens?

Quotes from J. K. Rowling are also allowed.

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The most important thing to mention is that spells aren't like software. You don't run them, and the same thing happens over and over again. There's a hidden component that we don't really get to see involving intention, will, power and practice. If a powerful, but inexperienced user casts a spell, it will have a markedly different effect than if an older, but less powerful wizard casts it. – Valorum Jan 16 at 21:01
    
Harry uses Ricto Sempra in the duel in the Chamber of Secrets film, not Expelliarmus. – ATB Jan 16 at 21:39
    
I m sorry. Snape uses Expelliarmus there. I have edited the question. – Lucharx Jan 16 at 22:04
    
@Richard: the Snape/Lockhard thing (movie version only) suggests that it also depends on who the target is. As in you can resist some magical spells to some extent - we already knew that was true for mental magic, re GoF and the Imperius curse, so perhaps it isn't too much of a stretch to suppose it also applies to Expelliarmus. (Perhaps with Lockhard being such a total doofus, his effort to resist the spell backfired and made it even more powerful than it would have been against someone who was completely unprepared?) – Harry Johnston Jan 16 at 23:38
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@leftaroundabout - They're like Microsoft software. – Valorum Jan 17 at 19:45
up vote 16 down vote accepted

It’s important to note that magical spells can vary in strength depending on users ability and power.

“Avada Kedavra’s a curse that needs a powerful bit of magic be- hind it — you could all get your wands out now and point them at me and say the words, and I doubt I’d get so much as a nosebleed. But that doesn’t matter. I’m not here to teach you how to do it.”

Fake Moody explains that the curses require strong intent, and magical power behind them to work effectively.

how Parvati Patil had produced such a good Reductor Curse that she had reduced the table carrying all the Sneakoscopes to dust.

Here we see Parvati performing a Reductor Curse that managed to destroy a whole table. Harry, on the other hand, had trouble making a hole through some dirt with a Reductor curse in the maze in Goblet of Fire.

Even earlier then this we see Lupin teaching Harry the Patronus charm.

“With an incantation, which will work only if you are concen- trating, with all your might, on a single, very happy memory.”

said Harry, quickly forcing his thoughts back to that first broom ride. “Expecto patrono — no, patronum — sorry — expecto patronum, expecto patronum —” Something whooshed suddenly out of the end of his wand; it looked like a wisp of silvery gas. “Did you see that?” said Harry excitedly.

Here we learn that the strength of the happy memory is what increases the power of a Patronus charm. We see that while a semi-happy memory creates a misty low powered patrons, with a very happy memory the spell works at full power and creates a corporeal Patronus.

Hatred rose in Harry such as he had never known before. He flung himself out from behind the fountain and bellowed “Crucio!” Bellatrix screamed. The spell had knocked her off her feet, but she did not writhe and shriek with pain as Neville had.

“Never used an Unforgivable Curse before, have you, boy?” she yelled. She had abandoned her baby voice now. “You need to mean them, Potter! You need to really want to cause pain — to enjoy it — righteous anger won’t hurt me for long — I’ll show you how it is done, shall I? I’ll give you a lesson —”

Bellatrix explains that you have to MEAN it for the curse to work effectively. Hermione and Ron’s confused queries about Travers and Bogrod, who were both now standing there looking blank. “I don’t think I did it strongly enough, I don’t know. . . .” And another memory darted through his mind, of the real Bel- latrix Lestrange shrieking at him when he had first tried to use an Unforgivable Curse: “You need to mean them, Potter!”

So now to circle back to Snape, he is an extremely accomplished wizard, one of the most powerful in the entire series behind Voldemort and Dumbledore. He both has the skill and power to amplify a basic curse to add extra force behind it. As we see just one year later, three disarming spells from Harry, Ron and Hermione have the same force behind them as Snape.

“Expelliarmus!” he yelled — except that his wasn’t the only voice that shouted. There was a blast that made the door rattle on its hinges; Snape was lifted off his feet and slammed into the wall,

Finally Snape intentionally uses far to much power on his Expelliarmus spell then needed to embarrass Lockhart.

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Oh wow, 1 minute apart. You gotta be quick ain'tcha? +1 – Au101 Jan 17 at 0:59
    
@Au101 haha i see we had the same idea – Himarm Jan 17 at 0:59

Bit of speculation regarding the scene in The Chamber of Secrets:

Snape is angry.

Snape's upper lip was curling. Harry wondered why Lockhart was still smiling; if Snape had been looking at him like that he'd have been running as fast as he could in the opposite direction.

Lockhart and Snape turned to face each other and bowed; at least, Lockhart did, with much twirling of hands, whereas Snape jerked his head irritably.

...

'On the count of three, we will cast our first spells. Neither of us will be aiming to kill, of course.'

'I wouldn't bet on that,' Harry murmured, watching Snape baring his teeth.

Harry Potter and the Chamber of Secrets - p.142 - Bloomsbury - Chapter 11, The Duelling Club

Emotions do affect the power and quality of spell casting in the series. Obviously, the Patronus charm is a good example of this, where a happy memory is everything. Riddikulus is arguably another example. Then, famously, there's the Unforgivable Curses. Barty Crouch Jr as Mad-Eye Moody has this to say of Avada Kedavra:

'Avada Kedavra's a curse that needs a powerful bit of magic behind it - you could all get your wands out now and point them at me and say the words, and I doubt I'd get so much as a nose-bleed. But it doesn't matter. I'm not here to teach you how to do it.'

Harry Potter and the Goblet of Fire - p.192 - Bloomsbury - Chapter 14, The Unforgivable Curses

Then you've got Bellatrix Lestrange on the Cruciatus Curse:

'Never used an Unforgivable Curse before, have you, boy?' she yelled. She had abandoned her baby voice now. 'You need to mean them, Potter! You need to really want to cause pain - to enjoy it - righteous anger won't hurt me for long - I'll show you how it is done, shall I? I'll give you a lesson -'

Harry Potter and the Order of the Phoenix - p.715 - Bloomsbury - Chapter 36, The Only One He Ever Feared

Beyond this general point about magic in Harry Potter, the effect of Expelliarmus is broadly very consistent - it even works on objects which aren't wands. I don't think there're two types of Expelliarmus or anything like that.

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When Harry and Malfoy are duelling in CoS, Malfoy flies across the room.

This is an invention of the movie. In the book version of Chamber of Secrets Harry doesn't once cast Expelliarmus at all during his "duel" with Draco Malfoy. Why they chose to portray it the way they did I have no idea (I've seen the movie once years ago), but it's likely because it made for a more dramatic scene than Malfoy's wand simply flying over to Harry.

However, there is sort of a precedent for it. At the end of Prisoner of Azkaban, Snape is sent flying against a wall, and subsequently knocked out, by Expelliarmus. In that particular case it was Harry, Ron and Hermione casting the spell simultaneously that did it. Combining the three spells was a lot more powerful, and had a less predictable effect, than a single wizard casting the spell. Contrast this with Lupin casting the spell alone, but on multiple targets, and the spell having the desired effect.

There's further evidence for spells having unintended - or at least different - effects when combined. Near the end of Goblet of Fire Harry, Ron, Hermione, Fred and George all curse Malfoy, Crabbe and Goyle at roughly the same time:

"Interesting effect," said George, looking down at Crabbe. "Who used the Furnunculus curse?"
"Me," said Harry.
"Odd," said George lightly. "I used Jelly-Legs. Looks as though those two shouldn't be mixed. He seems to have sprouted little tentacles all over his face. ..."

Generally, when a spell is cast by a single wizard, and not combined with other spells, the effects are portrayed consistently. It's only when multiple spells combine, or when the movies take liberties with canon, that you end up with different effects.

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He doesn't cast expelliarmus in the movie either. – ATB Jan 16 at 21:42
    
Yes I m sorry. Harry does nor cast expelliarmus there, but snape did. I m not sure, but I think in the book snape also cast expelliarmus against Lockhard. – Lucharx Jan 16 at 22:07

Out-of-universe, this is to demonstrate that Lockhard isn't just a blowhard but also a total doofus.

In-universe, I'm not aware of any explicit canon explanation for this incident. But if you just want a vaguely credible explanation, that's also because he's a total doofus.

We know you can resist or block magical spells to some extent, the most notable instances being in Goblet of Fire in which the class is taught to resist the Imperius Curse and Half-Blood Prince in which Snape blocks Harry's attacks, including physical spells such as Impedimenta.

It is reasonable then to suppose that a magical duel is like, well, a duel: if you are not fighting to kill or wound, you gauge the strength of your attack to the skills of your opponent. Snape, underestimating Lockhard's incompetence, cast Expelliarmus as he would have if dueling any other staff member; Lockhard was unable to resist, so the effect was greater than that intended. Lockhard's attempt to resist might even have backfired.

More likely still, Snape was probably perfectly happy to have an excuse to demonstrate Lockhard's incompetence!

An alternative theory can be based on Deathly Hallows in which we discover that there is a mystic bond between wizard and wand, and that wands may even be partly sentient. This makes me wonder whether Lockhard's wand may have been very eager to escape. :-)

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I don't really think, OOU, it's to demonstrate Lockhart is a doofus, I think it's to look cool. What happens to Lockhart is basically what happens to Malfoy when Harry uses Rictusempra (tickling charm) and what happens to Harry when Malfoy uses the (seemingly invented for the movies) Everte Statum spell. The duelling club scene is basically just guys going flying, doing cool somersaults all over the place. crude and tacky, but it's visually interesting and appealing to the target audience – Au101 Jan 17 at 0:33
    
@Au101: I was thinking of the book, not the movie. – Harry Johnston Jan 17 at 0:35
    
Yeah, fair enough, to be honest with you, I'd actually forgotten how exaggerated the effect was in the book. I still think it's more about the big effect, but actually you might well be right about him being made a fool of, and also about making the point that he was so outclassed. Fair enough, +1 :) – Au101 Jan 17 at 0:40
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@Au101: to be fair, "total doofus" was a little tongue-in-cheek. But either way I think it can be filed under "character development". :-) (Ah, I see you've already edited the scene into the question; I was just about to do that. Thanks!) – Harry Johnston Jan 17 at 0:56
    
@Harry Johnston Thank you for the answer. But I prefer the answer from Himarm now. I upvoted your answer, because I think the solution is an mix of both and I can't accept two answers =( – Lucharx Jan 17 at 9:52

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