If you look at video of a few hundred one-handed backhands frame by frame, you'll see that all the players straighten out the elbow early in the forward swing.
You can't look at a piece of moving video and see this. One thing to note is that if you "straighten" out your own arm--ie, fully extend the elbow--the shape of the arm is often still slightly curved. That's the case with Roger.
The arm and racket swing like a one piece gate from the front shoulder.
One of the few players who had trouble getting his arm straight early was Sampras. I have quite a bit of video showing that his errors on that side are related to this tendency. When he was late getting his arm straight, there are a far higher percentage of errors.
There are many different shapes and heights to the backswings--the elbow more or less bent, the racket tip pointing higher or lower, the hand higher or lower. But as the hand drops and the racket starts forward the straight hitting arm (accounting for the curve mentioned above) is a commonality.
In the modern game you now see an increasing number of wiper finishes on the one-handed backhand. When the players increase their hand and arm rotation, the entire hitting structure rotates as a unit. Sometimes in these more extreme finishes, you see some forward flex of the wrist--probably a passive consequence of the extra torque.
The Gasquet clip at the front of this thread is pretty blurry but that appears to be what is happening there.
We have to be careful though with generalizations based on one video clip. If you look at a player over the full range of shots and court positions and spins, then you have a better basis for a general understanding.