The ball starts with LW, who passes to the running in LB.
At the same time LW starts a running in behind LB.
LB returns the ball to LW, who immediately passes the ball to CB, who is moving in.
CB immediately returns the ball to LW.
LW now passes the ball to RW and the exercise starts over again from the right side.

The exercise can also be done in such a way that the running in LW - instead of passing to CB - passes to RB, receives the ball again and then pass to RW.