I cannot execute this shot myself, but know a couple of people who can. From what I understand, you step over the ball and do not hit it when it is between your legs. Rather, as your back is to the net, the ball will be behind your back and in the "hole" that you create when you spread your legs. So just manipulate the swingpath of the racquet so that you can get the head of it between your legs, and hit the ball while it is "behind" you on the other side of your legs. (Note: really the ball is "in front" of you and "behind" you at the same time. what i am trying to say is that the ball is closer to the net than you are, so really it is behind your back since your back is to the net.)