In order to get maximum racket head speed, as the racket drops, the elbow should remain at close to 90 degrees with the upper arm.
You must not bend the elbow in a scratch the back motion. The racket must not drop as a result of the elbow bending.
Instead you'll see that by keeping the elbow at 90 degrees, the racket goes around the shoulder as you turn the body.
Keeping the elbow at 90 degrees allows you to throw the racket with more force.
Fed 90 degrees
Roddick 90 degrees
Sampras 90 degrees
Warwrinka Bent elbow = loss of power
You are looking at videos, correct?
Moment of inertia is a defined quantity - it can be Googled. It indicates resistance to rotation. If the elbow is bent at 90 degrees and the racket is griped in the hand as it is, the moment of inertia is large so that if the shoulder mass is accelerated forward the masses of the forearm and racket will cause the upper arm to rotate backward with
external shoulder rotation (ESR). Watch those things occurring in a high speed video. It is difficult to describe in words as the body spins around and many sub-motions occur. But you can see them in high speed videos.
Look at other strokes, for example, forehands, where the
moments of inertia of arms and rackets are being used. When the shoulder mass is accelerated in one direction - the arm and racket with large moments of inertia lag behind - and stretch muscles, often shoulder joint muscles.
Stretched muscles store energy and elastic energy in stretched muscles
can shorten faster than active muscle shortening using Actin & Myosin in Sarcomeres. (I wish that I could find that stated in a biomechanics reference.)
As a demo of moment of inertia. Take a can of soup in your hand, elbow angle at 90 degrees and accelerate the shoulder mass forward and backward, gently, but feel the can of soup. Now put the can down. Do the same move and feel the difference. Moment of inertia around a specified rotation axis is found by considering each mass multiplied by the
square of its distance out from the specified rotation axis. So the can of soup should have a big effect and it does. That is why the forearm and racket are held as they are for the serve, for a larger moment of inertia. You ask why not hold both the forearm and racket shaft at 90 degrees for even more MOI? Because that is uncomfortable for the wrist joint with the angle of the grip.
Another demo is to use a heavy hammer. Hold it by the handle and pronate and supinate your wrist, side to side. Now hold the head of the hammer and repeat. That illustrates how the location of the mass affects resistance to rotation.
In addition to the use of moment of inertia with a 90 degrees elbow, as described, the body is bending and then straightening the spine and moving the scapula around. This also stretches or slackens the muscles that run across the back, see Thoracic Extension, Thoracic Flexion, and Scapulothoracic joint motions that change the muscle length, particularly of the lat muscle. Each of the defined joint motions can be Googled, with pictures and videos to show how to measure joint angles.
If you only look at few pictures and word descriptions, you will miss the things mentioned here.