Probably no single movement in the golf swing has more to do with controlled power than the movement and position of the upper right arm. The short arc the arm moves through as it nears the top of the backswing and then begins the downswing can be likened to the wing motion of a bird. It lifts up and away from the side at the top of the backswing, then settles back and against the side as the club proceeds through its downward arc.
It is not a tense motion. The right arm is not pressed against the side. But the upper right arm must return to a snug position against the side and remain there until the club head has moved through the ball. By being anchored there it provides a fulcrum around which the right forearm moves, thus providing the hitting power of the swing. Moreover, if the arm is returned naturally to the side as the body turns in the downswing, the golfer will be in balance through the hitting area.
It is the upper arm which controls the downward arc until it rests easily against the right side. Then the right forearm continues the arc of the swing until the club head is through the ball.
from GRAHAM ROSS, Dallas Athletic Country Club