Love that "wrist flick" term. Actually got into an argument at a presentation I was doing with none other than Johnny Mac about it years ago--suffice it to say he was unpersuaded by evidence, and is (still) one of the main progenitors of this highly misleading term! I showed him a video of Agassi's forehand with the wrist still laid back at contact. But it was like trying to tell someone the wall was blue when they were absolutely certain it was white... still love you John, no one is perfect.
And so that discussion has always gone...disagreement and more disagreement...the term I am pretty sure is destined for immortality. But I don't think it contributes much to understanding what happens in high level forehands.
If you study the players in high speed you see that for Fed, or Agassi, or Delpo, or Jo Willie, or any player with a moderate grip, the vast majority of all forehands are hit with the wrist laid back.
The angle of the lay back can change from the start of the forward swing and decrease at contact. But again in the huge majority of cases it's still laid back well after the hit.
Some players and coaches believe that this forward flexion is a key to power or spin and an example of that mythical, epic, invisible and misunderstood term the "stretch shorten cycle."
As I understand the work of Brian Gordon though this isn't actually correct--the real stretch shorten cycle is something happening in the shoulder muscles, as a function of body and backswing position at the start of the forward swing...
The wrist angle at contact, in reality, seems to be about positioning the racket head to hit the ball directionally. On a crosscourt from a wide position it will come around more, closer to neutral. In this sense it acts like a hinge.
When the hinge motion is substantial the movement can contribute to racket head speed. But if you look at inside balls--the majority of pro forehands--the wrist hardly flexes if at all and can actually become more laid back after the hit. So it's not some conscious attempt to flex or flick--more the natural relaxed movement of the joint as part of a specific swing path.
Now the other motion involving the wrist is the rotation of the hand, arm and racket. When this motion is somewhat to very extreme it's usually called the windshield wiper. But it's a component of all forehands to a greater or lesser extent. There is still rotation of the upper arm, forearm, wrist, and racket in counterclockwise direction on an old style "flatish" eastern forehand--think Tim Henman.
Where Federer broke the mold was in combining a grip that is still close to eastern with the radical hand and arm rotation of the more extreme forehands--a gorgeous blend of classical and modern.
And since the wrist is attached to the arm (one could only hope...) there is up to 180 degrees of counterclockwise rotation (or even more sometimes...) of the wrist in a big wiper motion.
Call that wrist flick if you wish. But it's being driven by the rotation of the upper arm in the shoulder joint. This rotation from the shoulder can be very fast--and that's why the wiper can generate so much racket head speed and why Federer can hit so much spin with that grip.
For skilled players who really do have the more foundational elements of turn and extension, the wiper is very viable situationally or even as more of a norm at higher levels. And now with poly this is more true than ever and even applicable at times at lower levels if those same fundamentals are in place.