I started doing this exercise to increase understanding between my wide forward in a front three and my centre forward. I wanted them to understand each other more in regards to the runs the striker makes and where he wants to receive the ball and it allowed the wide forward work on their final ball into the striker to meet his movement.
The drill works down the right then down the left. The opposite wall player started by comming in to apply pressure and this was progressed to just becomming a live defender when the striker takes their first touch.
Other possible progressionscould be:
-The wall player who plays the pass also becomes a defender and the centre mid supports the attack to make it a 2v2 to goal -If the defenders win the ball they should go and start an attack on the two small goals -Swap roles of players at the coaches discretion