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Single Arm Drill - Push off the wall with both arms extended in front of you with your hands touching, one on top of the other. Rotate to your side so that you are lying on your left side with your left arm in front on you. Now, swim down the pool, using only your right arm - once you get to the other end, try the same thing, this time lying on your right side and swimming with your left arm only. Keep a strong kick going and focus on the pull portion of the stroke.
The advanced single arm drill focuses more on the body rotation (see video).