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Serving underhand, crouch your body down as if you were going to bump. Have your right foot slightly in front of left. Make sure you're not standing up straight. Say you are right handed. Hold the ball out in front of you in your left hand. Pick a patch on the ball to hit. Make your right hand into a fist. Swing your arm back to shoulder height, and propel it forward like a pendulum. Some people prefer to hit it on the part of their arm where they bump it, because it can give more control, but it doesn't give as much power. Do what works best for you. Crouch, aim, and propel. That was easy, right? Make sure you don't bounce, throw the ball before, then hit it (you'll take out a ceiling light). You just need to hold onto it.
Position. Hands and arms. Do this move.