On all strokes, one essential component in the forward swing is the hitting arm position. This is especially so for the one-handed backhand: The hitting arm is straight at the start of the forward swing and stays that way all the way out through the extension of the stroke.
This basic point can get lost when studying the backswing. When we look at the pro players we see many of them have up to a 90-degree bend in the elbow at the top of the backswing. But this isn’t the key to hitting the ball like the pros. Copying the larger backswings with the most bend at the elbow is another cause of the dreaded “elbow lead,” in which the elbow comes through the contact first with the racquet head trailing behind. The result is late contact, loss of leverage, loss of power, and an inability to hit the ball crosscourt.
Virtually every pro player straightens his arm out completely by the time the racquet reaches the bottom of the backswing. This hitting arm structure is critical and the players maintain it all the way through the extension of the swing.