How did Harry learn Sectumsempra?

Question

All through the Harry Potter series we see that spells require specific techniques that need to be practiced; Wingardium Leviosa has to be pronounced exactly right (Hermione: "It's 'levioSAH'") and accompanied with a specific movement of the wand ("swish and flick").

In The Half Blood Prince, Harry encounters the name of the spell "Sectumsempra" with the cryptic comment "for use on enemies" and no other clues as to how to cast it or what it might do. Then in the duel with Draco he uses it for the first time to devastating effect. How come?

Answer 1

Harry apparently knew the incantation, but possibly not accurate inflection. It's possible that while, yes, the spell was intended 'for enemies' - it was not necessarily meant to be wielded with the complete devastating effect that Harry casts it with.

In the first book, we learn that inflection / tone / letter is very important in Charms. Professor Flitwick says:

"And saying the magic words properly is very important too - never forget Wizard Baruffio, who said 's' instead of 'f' and found himself on the floor with a buffalo on his chest."

When we see Snape cast Sectumsempra (book 7), he appears to have very targeted, almost surgical precision with it, though he strikes the wrong opponent:

...the spell, intended for the Death Eater's wand hand, missed and hit George instead -

Clearly - Snape was not meaning to obliterate neither the Death Eater or George, but meaning to hit a small target instead.

Harry, however, when he casts it (book 6), casts it wildly and without any precision at all, literally shredding Malfoy apart:

"Blood spurted from Malfoy's face and chest as though he had been slashed with an invisible sword...Harry got to his feet and plunged towards Malfoy, whose face was now shining scarlet, his white hands scrabbling at his blood-soaked chest."

The inflection/intention of how the spell is cast, details that may not have been included in the margins of Harry's book, could account for the discrepancy in how they affect the target.

Answer 2

He read the incantation and pronounced it well enough that it worked.

Although Harry didn't know what Sectumsempra did, and didn't have explicit instructions how to cast it, he had seen it written out in the Half-Blood Prince's book.

There are at least two ways Harry could have managed to cast Sectumsempra without ever getting told how to pronounce it. Sectumsempra could have been easy enough to figure out how to pronounce by just seeing it written - certain words are easy to figure out how to pronounce without ever having heard them, and the same thing might apply to spells. In addition, he might not have pronounced or cast it perfectly, just well enough for it to have that effect on Draco.

If Harry was being graded on his casting of it, he may well not have gotten the top grade, he might have even failed. It had an effect, but likely not the exact effect that Sectumsempra has when cast perfectly.

Some spells don't seem to require special wand movements, so Sectumsempra might not.

Wingardium Leviosa requires a specific wand movement, but that doesn't seem to be the case for all spells, even more difficult ones. Some spells seem to be more 'point and cast', since specific wand movements aren't always shown being taught or used.

Lumos, for example doesn't seem to require a specific wand movement.

“Lumos,’ Harry muttered, and a light appeared at the end of his wand, almost dazzling him. He held it high over his head, and the pebble-dashed walls of number two suddenly sparkled; the garage door gleamed, and between them, Harry saw, quite distinctly, the hulking outline of something very big, with wide, gleaming eyes.”

Neither do Crucio and Imperio, since Harry casts both without ever finding out specific wand movements for either of them.

Answer 3

In terms of correct pronunciation:

1) Most spells based on non English words in the Harry Potter Universe are based on Latin, examples being accio, confundo, crucio, imeprio, diffindo, lumos, nox, Homenum Revelio, Priori Incantatem.

Harry knows English, and doesn't recognise the words Sectum and sempra. Therefore, he can make a pretty safe guess that they are probably Latin.

2) The words ARE from Latin. Roughly meaning "having been cut, always".

Therefore, Harry attempting to pronounce them as Latin words in on the right track. He also knows that it is for cutting your enemies. This may unconsciously bias the wand action used.

3) English is renowned for being inconsistent, making it hard to pronounce a written word. Latin is not.

Therefore, a competent Latin speaker could pronounce the word correctly, having seen it written.

4) Harry has been educated in the pronunciation of Latin words, and Latin based, in a way that makes it very easy to know when you are right or wrong.

Therefore, Harry can pronounce the words correctly.