This is why your cat shakes his bum

Wiggle wiggle wiggle, you must have seen your cat shake his butt. The wiggling with the back often precedes the prey. In this way your cat can feel whether the ground on which he is depositing is strong enough. The cat also tightens his muscles before he makes the big leap. You will also see that your cat keeps its head still while it is doing this.

Here is wiggle cat movie (watching cat movies is very healthy for you, scientific research shows).