The Baseball Diamond (Hand/Arm Rotation) Drill
A good drill to practice a proper hand and arm rotation would be to imagine you are hitting golf balls out of the batters box on a baseball diamond. Hit the ball and try and make sure when the club is parallel (90 degrees) to the ground after impact (Image 1), the club head, shaft and right arm are all pointed in between the pitchers mound and where the second baseman stands (Image 3), Highlighted Blue Area. Your right forearm should also be pointed straight up to the sky at this position (Image 2).
Image 2 illustrates how the right forearm has rotated on top of the left and the back of the right hand is pointed up to the sky.