Keeping your head directly over the ball during the backswing causes you to shift your weight forward, instead of loading up on your back leg, causing a slice or hook
The creation of power in the golf swing comes from balance, the body, or leverage, the arms, use them both on the downswing to create the maximum amount of power in your swing
Regardless of trajectory, always use the same stance, setup and grip; just change your ball trajectory and swing length