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.