The ball starts with LW, who pass the ball to the attacking LB.
At the same time LW starts a running in behind.
LB passes the ball to the attacking CB, who sends the ball to LW, who is running behind.
LW immediately passes the ball to RW and the exercise starts over again from the opposite side.

The ball may also be returned to RB, who immediately sends it to RW.
RW passes the ball to RB and the exercise starts again from the opposite side.