If B, playing the woman on top, leans forward to offer both her and A more physical contact, A can thrust by pushing his or her hips up off the bed using his or her legs. In this position, A doesn't need to have a lot of upper body strength, but does need strong leg muscles and hips.
This is an example of how the woman on top position doesn't always mean that the person on top does all the work. Here the person on top is changing the position and this allows the person on the bottom to take a turn at thrusting and creating the movement.
