1. Not only is there a lot of movement within each shot, from the actors particularly, but the shots also change in rapid succession, moving the point of view of the audience around the scene.
2. By distorting certain visual elements, like the keys and the dancers, Hitchcock shows us how the mind of the protagonist is essentially getting 'bent out of shape.' I love the use of the dancers as a mini boxing match - when they get exhausted and fall into their separate corners to be fanned, then pushed back into the centre of the floor, not to fight, but to dance.
3. The use of overlay, with the image of the musical instruments over the protagonists head as he gets upset, shows how the sound is making him crazy.
*SIDE NOTE: I'm not a huge fan of the music used here. It's only piano when clearly other instruments can be seen - which is fine, but it's best if the piano tries to imitate or give an impression of the other instruments, which isn't done here. Also, the music slows down just as they show the musicians going faster and the protagonist getting more worked up - this works against the Hitchcock thrust of the whole scene.