Rub a dub dub!

When cats roll over and show their bellies, they do so because they entrust you to their most vulnerable spot on their body (even though it’s surrounded by 18-20 daggers)

As kittens, they expose their bellies to their siblings, whom then tackle that kitten’s belly. It’s a form of play between each other. This is why your cat attacks your hand when you reach down to pet her belly.