The notion that you can volley effectively without changing your grip between your forehand and backhand volleys is somewhat inaccurate.
Although many professionals volley without rotating the grip between the thumb and forefinger, there is a small change of alignment between the grip and the hand.
On the forehand volley, the racquet grip is usually aligned with the lifeline of your palm.
On the backhand volley, as you push your racquet hand away to take the racquet to your backhand side (as if looking at the face of your wrist watch), the grip moves slightly inside your hand, crossing it in a bit more of a perpendicular way. The left hand (for a right-hander) aids in taking the racquet to your side.
In other words, the butt of the racquet points to the ball, but the grip change was so slight that the racquet face is open and laid back, facilitating a simple move to strike the ball.
This grip rotation is instinctive and born from practice, with the player adopting, by feel, the most efficient way to hit the ball.
The best forehand volleys are hit across the body rather than forward. They are a very short downward motion, with the racquet face slightly open, and they firm up at impact with the ball. I call it a one inch punch. The direction and depth of the placement depend on where the racquet angle is aimed.
While on groundstrokes it is best to get below the ball and hit up, the opposite is true on volleys. You should hit down, as if your racquet was going to the bottom of the net, but with a short, chopping hit.
The racquet face should be open according to the height of the incoming shot.
The best way to learn to copy the pros on the backhand volley is to advance the butt of the racquet across the body, from left to right, as if you were elbowing somebody out of the way. On the forehand, as if you were stopping the butt of the racquet in front of your belly with the palm of your left hand.
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