Korean Pro Golfer Heo for the win.
The Wrists in the Golf Swing
If one wants to understand supination, radial/ulnar deviation, flexion, and how the clubface is controlled by the hands/wrists refer to Homer Kelley's book "The Golfing Machine". <---Mac O'Grady and Gary McCord go nuts on TGM. 99% of hacks have no interest nor any idea what this is about nor do they care. BUT, all pros do these factors some of them somewhat differently, but ALL do them very well. In fact, I'll go out on a limb and say 80%+ on this forum have never felt what Marty Nowicki shows in the use of Kelvin Miyahira's invention the Impact Snap. The Impact Snap puts your hands, wrists, arms in these positions which like I say most have never felt. BTW, there is no right or wrong if you don't like this stuff just pass over it. This is for folks that want to understand more. The OP asked a question so here's more info. that many have not even considered.
I practice swinging occasionally with a 90 degree wrist brace. I don't think it will ever happen, but I want every thing to be in the same place at the same time every time.
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Your wrists are levers. They’re being used in the golf swing (for good or bad) whether you consciously feel it or not. No one can swing the club without the wrists involved. Even Steve Stricker.
I see lots of instructors teach supple wrists. Not sure where you'd get the idea it's rare.
If you want to be loosy goosy in the wrists great as long as what ever you do in your swing is repeatable.
garyt1957 I see lots of instructors teach supple wrists. Not sure where you'd get the idea it's rare.
I've been to 6 different instructors over the years. Nobody told me anything about wrists, except "don't cup at the top." Would have been nice if I was told "here's something you're not doing that can add 20-30 yards." I guess I saw the wrong instructors! 
Have you hit enough balls in your lifetime that you can say you've never done that since it was pointed out?Oh, got it. I was told not to cup when I first started playing in the ‘90s. Wanted to get rid of my slice, and so I definitely practiced it (along with fixing my swing path). Eventually I became a hook machine and I stopped purposely trying to flatten the wrist.
Interestingly, I stopped playing regular golf for about 12 years (because of time, kids, expense, etc.). Picked it up again in 2018, but rarely fade/slice, so cupping is not something I thought about. I do not see cupping when I look at my swing on video, so I guess it was ingrained.
The biggest change I found when picking up golf again is lost distance.
4 days later
Phillip Sparks posted this on 8/10. Worth a look especially for seniors with lost distance.
Eguller That's basic Manuel de la Torre. There is much aligned with MDLT/Ernest Jones and Sparks/Easiest Swing. Free wrists, centered ball position and NOT a body-oriented swing. When my game goes off the rails this is the main reason why also why I can play golf at 75 without any body part replacement so far. 
And still walk and carry 7-8 clubs for 9.
Eguller For most golfers, it takes trust, practice and a solid mind set to execute this swing with confidence in ‘tournament play’. The urge to HIT under those conditions is almost irresistible.
Agreed Dan! It takes a solid mental game to play golf hence the issues with majority of hitting from the top hackers. I was lucky I got slapped on the ass from Manuel at 12 trying to hit at the ball. I'm still a hacker but I understand why, the majority of golfers never understand the why's. And therefore can never "fix" themselves.
Basically, IF I follow thru to a full finish the resulting shot will be 90% good to great. To me it's the end to the means. ("something that you do because it will help you to achieve something else") Free wrists and low tension is all part of that. Low tension stops that temptation to hit at the ball.