If the racket and the elbow move back together, then the lateral muscles at the back of the arm are largely in control.
When you swing forwards to make contact with the ball then these same muscles will be doing the work.
The problem is that the lateral muscles are the weakest part of the arm.
If the racket is layed back and the elbow remains by the side of the body, then the racket is further back than the elbow. This results in the medial muscles at the front of the arm (including the bicep) are now largely in control of the movement.
The swing will now remain under control but have much more force and power available.
This technique is great for all strokes, but in doubles can be very intimidating for the opponents
See what happens when you ignore traditional tennis advice and start to “Swing at Your Volleys”