The exercise trains the triangle-movement in the defense.
A presses forward and C moves forward in defense.
A passes the ball to B, who press forward.
C moves diagonally backwards as a backup for D, who moves forward to cover B.
B returns the ball to A, who in the meantime has moved back to her starting position.
D moves at the same time diagonally back towards C, who simultaneously with the ball pass has moved back to her starting position.
A press forward and so on....
The exercise has a very high intensity and must not be repeated too many times.