The defender should “turn and clamp,” “getting a piece” of the picker (with two hands on their stick) and running “hip-to-hip” with them as they “roll” to the net.  The offensive player with the ball then comes around the pick (looking like they want to shoot) and attempts to force a pass into the roller (Variation A) who tries to catch it and finish in tight.  Another option is for a coach to be waiting and playing dummy defense (so players get a feel for the "switch" that generally happens in this scenario), lightly contesting the pass into the roller (Variation B).

The next time through the drill, the picker becomes the defender, the defender joins the offensive line they just defended, and the offensive player who just passed the ball to the roller now joins the uncovered offensive line (with balls).

The purpose of the drill is for learning to anticipate and react to picks.