I call it "elevator butt," although the technical term for the posture is lordosis. Many cats place front paws and tummy on the ground, while the tail and ass-ets rise to the occasion.
The posture appears to be an invitation to scratch that hard-to-reach spot right at the base of the tail. Seren routinely does this, and acquires a look of kitty bliss during the scratching.
Why Cat Stick Butts In Air
In fact, lordosis posture IS an invitation by intact girl cats to entice the hunky boy cats to get romantic with them. I'm being facetious, but this is the posture females in estrus use to invite mating. This is the kitty version of wearing a low-cut sexy gown.
Male cats sometimes get creative when baptizing with urine and assume the position. That apparently improves their aim, and makes the pungent message land higher on the target. Maybe the fellas believe that makes them appear even bigger and more impressive.
But many spayed or neutered cats practice the elevator butt pose around humans. No, they do not have romantic intentions toward us. Still, the posture places the cat in a vulnerable position, often with the tail held high in the universal feline friendly gesture.
Elevator butt is a declaration of friendship, and an invitation to come closer and interact. By scratching or petting your cat in response, you're reinforcing (rewarding) the behavior, and also telling the cat you accept the offer of friendship.