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?