It is.....also turning their rear to you and doing the quivering thing with the tail. Since you have more than one, you know how they sniff each other's tushie, and apparently cats only do that with another cat that they are familiar with. When they come up to you (like when you are sitting or laying down) and stick their tush towards your face....it's cause they want you to know where they have been. (like you don't already know
)
I love that cats and dogs are so uninhibited. They ALL go around sniffing each others' butts. The elevator butt of a cat, so I've read, is their signal that you are, unworthy human though you are, are good enough to scratch their back.
It's quite a sight to watch one of my kitties constantly chewing on the toes and tail of the four dogs. Wouldn't believe it someone told me, but it's true, he is fearless!