What magic forced Uncle Vernon to suddenly let go of Harry as though shocked?

Question

I'm curious about what's going on in this scene.

A loud, echoing crack broke the sleepy silence like a gunshot; a cat streaked out from under a parked car and flew out of sight; a shriek, a bellowed oath, and the sound of breaking china came from the Dursleys’ living room, and as though Harry had been waiting for this signal, he jumped to his feet, at the same time pulling from the waistband of his jeans a thin wooden wand as if he were unsheathing a sword. But before he could draw himself up to full height, the top of his head collided with the Dursleys’ open window, and the resultant crash made Aunt Petunia scream even louder.

Harry felt as if his head had been split in two; eyes streaming, he swayed, trying to focus on the street and spot the source of the noise, but he had barely staggered upright again when two large purple hands reached through the open window and closed tightly around his throat.

“Put - it - away!” Uncle Vernon snarled into Harry’s ear. “Now! Before - anyone - sees!”

“Get - off - me!” Harry gasped; for a few seconds they struggled, Harry pulling at his uncle’s sausage-like fingers with his left hand, his right maintaining a firm grip on his raised wand. Then, as the pain in the top of Harry’s head gave a particularly nasty throb, Uncle Vernon yelped and released Harry as though he had received an electric shock - some invisible force seemed to have surged through his nephew, making him impossible to hold.
_{(Order of the Phoenix, Chapter 1, Dudley Demented).}

The other elements of this incident are explained later in the book. The crack is Mundungus Fletcher Disapparating. The cat belongs to Mrs Figg and is keeping watch on Harry/Mundungus. The shriek and the broken china come from the Dursleys themselves.

Yet the force that makes Uncle Vernon suddenly let go of Harry is more difficult to fathom. It's possible that it was a normal electrical shock as a result of static. However, it seems far more likely that there was something magical going on. Vernon had a hold of Harry for several seconds before releasing him so it clearly wasn't that he couldn't touch Harry at all (as was the case with Quirrell). I'm not clear on whether or not the magic was connected to the pain in Harry's head.

What exactly caused Uncle Vernon to suddenly let go of Harry?

Answer 1

Instinctive, uncontrolled magic.

There are lots of cases of Harry unknowingly using magic to defend himself. For example:

Once, Aunt Petunia, tired of Harry coming back from the barbers looking as though he hadn't been at all, had taken a pair of kitchen scissors and cut his hair so short he was almost bald except for his bangs, which she left "to hide that horrible scar." Dudley had laughed himself silly at Harry, who spent a sleepless night imagining school the next day, where he was already laughed at for his baggy clothes and taped glasses. Next morning, however, he had gotten up to find his hair exactly as it had been before Aunt Petunia had sheared it off. He had been given a week in his cupboard for this, even though he had tried to explain that he couldn't explain how it had grown back so quickly.
_{Harry Potter and the Sorcerer's Stone, chapter 2}

...

Another time, Aunt Petunia had been trying to force him into a revolting old sweater of Dudley's (brown with orange puff balls) -- The harder she tried to pull it over his head, the smaller it seemed to become, until finally it might have fitted a hand puppet, but certainly wouldn't fit Harry. Aunt Petunia had decided it must have shrunk in the wash and, to his great relief, Harry wasn't punished.
_{Harry Potter and the Sorcerer's Stone, chapter 2}

...

On the other hand, he'd gotten into terrible trouble for being found on the roof of the school kitchens. Dudley's gang had been chasing him as usual when, as much to Harry's surprise as anyone else's, there he was sitting on the chimney.
_{Harry Potter and the Sorcerer's Stone, chapter 2}

...You get the point. He has a history of using unknowingly using magic to defend himself. He does it again in The Prisoner of Azkaban:

“MARGE!” yelled Uncle Vernon and Aunt Petunia together as Aunt Marge’s whole body began to rise off her chair toward the ceiling. She was entirely round, now, like a vast life buoy with piggy eyes, and her hands and feet stuck out weirdly as she drifted up into the air, making apoplectic popping noises. Ripper came skidding into the room, barking madly.
_{Harry Potter and the Prisoner of Azkaban, chapter 2}

And magic does help make sure that the wizard doesn't get hurt, as we see with Neville:

My great-uncle Algie kept trying to catch me off my guard and force some magic out of me – he pushed me off the end of Blackpool pier once, I nearly drowned – but nothing happened until I was eight. Great-uncle Algie came round for tea and he was hanging me out of an upstairs window by the ankles when my great-auntie Enid offered him a meringue and he accidentally let go. But I bounced – all the way down the garden and into the road. They were all really pleased. Gran was crying, she was so happy. And you should have seen their faces when I got in here – they thought I might not be magic enough to come, you see. Great-uncle Algie was so pleased he bought me my toad.
_{Harry Potter and the Sorcerer's Stone, chapter 7}

And, this line appears to be told from Vernon's perspective:

some invisible force seemed to have surged through his nephew, making him impossible to hold.
_{Harry Potter and the Order of the Phoenix, chapter 1}

So he literally can't hold him.

So it was probably a case of uncontrolled magic.

Answer 2

Well it's a matter of interpretation really. It's very possible that, (as Mithrandir suggests in the comments (and I hope they will make it an answer)) it was some kind of unspecified instinctive magic.

I, however, always thought it was a bit of a joke. Well not quite a joke exactly, but a bit of playfulness. Here's what comes next:

Panting, Harry fell forwards over the hydrangea bush, straightened up and stared around. There was no sign of what had caused the loud cracking noise, but there were several faces peering through various nearby windows. Harry stuffed his wand hastily back into his jeans and tried to look innocent.

Harry Potter and the Order of the Phoenix - p.10 - Bloomsbury - Chapter 1, Dudley Demented

I always thought that we were meant to think it was magic, until we realise it's Uncle Vernon's age-old fear of what the neighbours might think that has had an almost magical effect on him.

Personally, I think the construction of the paragraph does point to this interpretation. There's no explanation for the first mystery - the loud cracking noise - but there were people looking. I read this sentence as saying, in part: "We still haven't resolved the first question, but we can at least answer the second one: why did Vernon let go?"

Furthermore, look at the next two paragraphs and how Vernon immediately tries to cover himself after letting go of Harry:

'Lovely evening!' shouted Uncle Vernon, waving at Mrs Number Seven opposite, who was glaring from behind her net curtains. 'Did you hear that car backfire just now? Gave Petunia and me quite a turn!'

He continued to grin in a horrible, manic way until all the curious neighbours had disappeared from their various windows, then the grin became a grimace of rage as he beckoned Harry back towards him.

Harry Potter and the Order of the Phoenix - p.10 - Bloomsbury - Chapter 1, Dudley Demented

Note also that he doesn't show any signs of pain or discomfort or of being hit by a spell. Of course that could just be the nature of the undefined magic used, but I find it quite indicative.

Answer 3

My interpretation is that it's uncontrolled magic that makes Harry impossible to hold. He's got his wand out, after all, something he would never normally do in Privet Drive (because he's been startled by the loud crack.) He's preparing to defend himself, when Uncle Vernon tries to stop him.

Uncontrolled magic happens in the following year when he blows up Aunt Marge, again in an extremely emotionally stressful moment,and is clearly not limited to early childhood.

The loud crack is the noise of an elf apparating or disapparating, and sure enough it turns out that Dobby's on the scene. Wizards and witches seem not to make so much noise about apparating.

Answer 4

I'm a bit rusty on Harry Potter trivia and it's been a while since I've read the books. I stumbled upon this question while browsing for programming fixes on stackoverflow so don't take this answer too seriously. That being said, the first thing that struck me when I read the lines..

Then, as the pain in the top of Harry’s head gave a particularly nasty throb, Uncle Vernon yelped and released Harry as though he had received an electric shock...

..instantly made me think that perhaps the throbbing was actually from his scar, though he wasn't able to pinpoint it at the time due to the dulled sensation as a result of being in a choke hold. So his mother's love embedded in the scar, which is what saved him for Voldermort comes to his aid as an auto-executing spell when he needs it.