The Crab sex position is a receiver-on-top position that is a variation of the Cowgirl position.
To get into this position, the receiver's legs are bent in front of them on either side of their partner while they lean back. The receiver then supports their weight on their arms behind them. In this postion, the receiver is supporting most of their own bodyweight. To help, their partner can bend their knees slightly so that they can rest some of their weight on the knees, rather than supporting themselves fully.
More About Crab Position
This sex position should be eased into when the penetrator is penetrating with an appendage that has bloodflow because there is an increased amount of strain in this position. Once in position, the receiver uses both arms and legs to move themselves up and down on their partner.
They can also grind on their partner by thrusting their hips backwards and forwards. This should be done slowly so that the penetrating partner is not injured. This movement specifically targets the G-spot. The penetrating partner can also hold the receiver's ankles to prevent them from sliding forward.