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Vernon Dursley had entered the room, Petunia at his shoulder and Dudley skulking behind them both.

"Yes," said Dumbledore simply, "I shall." He drew his wand so rapidly that Harry barely saw it; with a casual flick, the sofa zoomed forwards and knocked the knees out from under all three of the Dursleys so that they collapsed upon it in a heap.

Harry Potter and the Half-Blood Prince

I don't quite understand "knocked the knees out from under all three of the Dursleys". I can't picture it. How should we understand it here?

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The Durstleys are standing in front of the sofa. Dumbledore makes the sofa quickly go forwards, so it hits them in the back of the leg, making their knees bend.

If you are hit in the back of the knee, and you aren't ready for it then you legs will bend and you will fall down in a sitting position. This is called "knocking the knees out from under someone". So the sofa hits them and they all fall down onto it, in a rough sitting position.

  • Thanks! Is it a common expression used by native speakers? – dan May 30 at 13:12
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    Not really common. I suppose you might use it to describe a wrestling move. It is more a literal description than an idiom. There are various expressions which end in "... from under something", usually suggesting "removing support". – James K May 30 at 16:06

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