Everyone has heard the tip about tucking the right arm against the side in the downswing, but few people know why it's so effective. It works because it drops the right shoulder, putting the club on the correct path into impact -- from slightly inside the target line -- for more solid and consistent contact.
If the right arm stays away from the side, the shoulders turn horizontally, causing an outside-in swing path and weak slices.
A good way to groove the correct move is to keep your back to the target longer on the downswing. This allows your right shoulder to drop and your arm to tuck against the body, a strong position from which to deliver the club through impact.