This Ball Kicker project synchronizes the action of the NXT motors at its two joints ("hip" and "knee") to produce a crack-the-whip effect to give the ball a good whack.
The simple NXT-G program shows two basic ways to synchronize the action of two motors that are not acting at exactly the same time. On the retraction of the leg, different motor power levels are used to allow one motor to go farther in the same amount of time, and on the forward kicking stroke, one motor waits on the rotation sensor of the other motor before starting so that both motors can operate at full power and finish at the same time even though one motor must start first to go farther.