Interesting article from one of my cat guides:
Your cat may approach you, turn around, and present his bottom to you as if to say "ya wanna sniff me?" This is something he only invites his friends to do, whether the friend is human, feline, or even canine. Some experts say the kitten is offering an invitation to take a sniff so you'll know who he is, sort of a cat hand shake. Others say you remind him of his mother, and he is asking you to groom his bottom. I do not recommend you go to that extreme to bond with your cat, but thanking him and scratching the base of his tail when he presents his butt is an acceptable response.
Your cat may approach you, turn around, and present his bottom to you as if to say "ya wanna sniff me?" This is something he only invites his friends to do, whether the friend is human, feline, or even canine. Some experts say the kitten is offering an invitation to take a sniff so you'll know who he is, sort of a cat hand shake. Others say you remind him of his mother, and he is asking you to groom his bottom. I do not recommend you go to that extreme to bond with your cat, but thanking him and scratching the base of his tail when he presents his butt is an acceptable response.