Not every cat does it, in my experience. Your domestic cat prepares to attack and pounce on prey. Or your cat might be about to chase a piece of string that you are pulling along. Your cat lies low to the ground, poised. She might raise her hindquarters slightly in the air and wiggle her butt (UK = bottom). Then within about 2 seconds she immediately pounces on the bit of string that you're pulling along. What is the reason for the feline butt wiggle? What is its purpose?
There can only be 2 reasons: either mental or physical preparedness for a quick dash to attack prey. It may be a way of priming the cat's hindquarters in readiness for a sharp contraction of the muscles of the hindquarters which then propels the cat forward rapidly.
I think, though, that the physical aspect of this form of cat behaviour is secondary to its role in mental preparedness for what could be a difficult task. In the wild when a cat attacks pray there is an element of danger for the cat. In the wild, wild cat species attacking large prey can be killed themselves in the process. Or, the wild cat can be hurt badly enough so that eventually he starves to death because he is unable to make an effective attack on prey.
So an attack on prey is a potentially dangerous process and therefore the cat has to prepare mentally for it. The feline butt wiggle builds up mental momentum, it winds up the cat mentally to a point where he is totally focused and committed to the attack.
I would liken it to the way that tennis players bounce the tennis ball before making a service or golfers waggle the club before making a drive from the tee and the way that athletes take a deep breath before embarking on a high level of mental and physical exertion.
That is my personal analysis of the cat but wiggle. I may be wrong. If anybody has a better idea then please leave a comment. Thanks in advance.