Sacral dimples

From Voyeurweb's Wiki

A sacral dimple is an indentation, present from birth, in the skin on the lower back. It's usually located just above the crease between the buttocks. Also called pilonidal dimples, most sacral dimples are small and shallow.

Due to their position on the lower back, sacral dimples are sometimes called thumb rests as they offer perfect positioning of the thumbs during doggy style sex.