A cat’s most vulnerable area is her/his underbelly, and therefore they can be very protective of it. Once they trust you they’re more likely to get on their back and expose themselves.
Though not shown here, our cat Lojo sometimes sprawls out on his back with both front and back legs stretched out and his belly completely showing.
After recently experiencing multiple men mansplaying on NYC subways, Lojo appears to be doing the feline version, therefore cats-playing.