Myth - You should extend your arm straight out and up pointing toward your target on your groundstrokes.
Truth - The pros swing upward, but do not exaggerate swinging outward like conventional methods teach. In fact, the pros swing so much upward that their body leaves the ground to maintain their balance and produce even more power and topspin (topspin makes the ball rotate forward allowing you to hit the ball higher above the net letting the spin pull the ball down into the court)..
On the forehand the pros bend their elbow on the follow through as they swing over their shoulder or just below the shoulder. This is a natural motion and produces topspin on their shots.
On the backhand you should have a relaxed, high follow through that is also upward more than forward. Do not try to force your follow through to come to a screeching halt on the backhand. Just let it go!
Conventional methods try to fit you into a mold that is not natural and can hamper the instinctive, automatic, and spontaneous play that is the signature of a champion. Learn to be more relaxed and natural with your follow through.