If you watch Prisoner of Azkaban, there's a scene where Snape gets hit so hard he's out for the count. It wasn't even a high-impact spell like Stupefy or Reducto—it was just a basic Expelliarmus.

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The movie depicts it like Harry was just mentally overwhelmed and couldn't control the power, but the novel goes deeper into the vibes.

Snape: 'Ha! Gotcha, you punks! Daring to kidnap and try to kill students?! Get ready for a one-way trip to Azkaban!'

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Harry: 'Professor, wait! They aren't trying to kill me! Sirius is innocent, and if Professor Lupin wanted me dead, he would've done it long ago during our 1-on-1 DADA lessons!'

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Snape: 'I caught them red-handed, Potter. Defending your own kidnappers? You've clearly lost your mind. I'm arresting these bastards first.'

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Harry: 'Just listen to me for a second, damn it! I know you guys had beef back in the day, but aren't you just doing this for petty revenge?'

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Snape: 'Silence, Potter! I won't tolerate that! Like father, like son—just as brain-dead as your old man! You think you're still alive because you're actually good at magic?!'

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Snape: 'You'd be dead if it wasn't for me, you moron! You have no idea what I’ve endured because of you! You arrogant prick! I know you think you're always right because your head is so far up your own ass! You're the spitting image of your father. Now shut up and stay back, you idiot!'

(Harry is fuming)

Expelli-fucking-armus... and that's how it happened. Of course, Snape still protected Harry with his bare body later, even without a wand.