The reason most instructors tell newcomers to lock their wrists is to get them to STOP the rod quickly so that it "unloads" and "flicks" a tight loop of line in the intended trajectory.
Without this instruction, most newcomers use their wrists in a "windshield wiper" motion and either never form a loop at all, or form one so big that the fly, leader and line all land in a pile in front of them after hitting the ground or water behind them.
Also, when using the thumb on top grip on the backcast, any wrist movement backwards will lay the rod down horizontally when the forearm reaches a vertical position.
With the backcast in particular, and with medium weight rods, most people's wrists are not strong enough to generated the rod speed required for a long backcast.
Below are two snapshots taken from a video of a presentation backcast with an 8 wt sage TCR rod strung with a 7 wt WF line. This backcast (like all of my backcasts) is done with a palm forward grip.
Notice the position of the reel, jammed against the forearm. No wrist motion involved whatsoever. Notice the angle of the forearm in all three pictures below.
This one is just milliseconds prior to the stop.
The picture below is just past the hard "STOP". You can see the reel lifting off the forearm as the rod is unloading.
The one below is during wrist relaxation to "Dampen" the "Rebound" after the "counterflex". Note the position of the reel in relation to the forearm in the above two pictures. Since this is a presentation cast, the wrist is being rotated so the rod can be comfortably lowered. If seen during the video it does not appear that there was ever a stop at all.
Once you get the hang of squeezing to a " hard stop", that stop can immediately merge into a relaxed wrist follow through. But without a hard stop (however brief) there can be no "power snap" as Joan Wulff describes it.
You simply have to experiment with your stroke to develop your own style of casting. On the final presentation cast, your hard stop on long presentation casts may wind up being when you run out of arm length.
---------- Post added at 12:22 PM ---------- Previous post was at 12:02 PM ----------
Wow! Lots of posts while I was typing and answering phone calls.
Lambster and I tried to get him to tighten up the wrist some, but it was so engrained in his casting he couldn't do it.
That is where the "forefinger directly on top" grip comes in handy. It is harder to drop the rod too low on the backcast using that grip, even with a limp wrist. It is sort of an enforced, (very uncomfortable) palm forward grip.