(Best not read if you’re eating!)
One of the delights of cat motherhood is washing a newborn kitten after its fed. The action of the mother cat’s tongue on the kitten’s belly and bottom causes it to relax and release pee and poo. This is then hoovered up by the mother cat. The equivalent of a human nappy change (but without the nappies)!
In the wild, getting rid of the waste in such a scrupulous manner is a precaution to make sure that predators don’t pick up the scent of the kittens. It also keeps the nest clean.
Here is a video of Ananda cleaning one of her newborn kittens [takes you to YouTube].
Some kittens must find it ticklish or intrusive because they squeal and try to get away. Mum then pins them down with a paw and just carries on doing the business.