Search here and you'll find several posts that refer to the wrists in the full golf swing; there's even a previous post on wrist action at the top of the swing, and I wanted to expand on that fundamental quite a bit in this post.
First, it's important to make a few concepts clear. Many wanna-be golf instructors (and some legitimate ones) tend to use "hinge" and "cock" interchangeably when referring to the wrist action in the golf swing. The wrists actually cock up and down in the golf swing (more correctly called radial and ulner deviation), and the right wrist also hinges backwards or dorsiflexes on the backswing. The wrists should up-cock on the backswing (radial deviation) and down-cock on the downswing (ulner deviation); the right wrist should delay straightening from its hinged position; and the left wrist should NEVER dorsiflex or hinge backwards during the downswing (also known as a flip).
It's important to allow the weight of the golf club to cock the wrists vertically at the top of the backswing; this explains why so many golf professionals say that you should learn to feel the club head and use light grip pressure with soft wrists. Doing this effectively sets or cocks the wrists (especially the left wrist) at the top naturally. The feeling of the left wrist setting or loading at the top (due to the weight of the club head) should be felt at the base of the left thumb and the side of the wrist (as the left thumb somewhat supports the shaft) and felt in certain forearm muscles. TGM circles call this using Power Accumulator #2. This feeling of loading the wrists (sometimes called float loading) can be used as a trigger to begin the downswing weight shift; doing it this way will serve to keep the lag well into the downswing and helps provide that whip-like feel at the bottom. When one gets to the top, there's a sense of everything collecting, as the club should be the last thing moving before and during the transition. No conscious effort to hold lag is needed, because you'll be delivering the lag you loaded at the top. Finally, it's not necessary to rush down when you feel that load in the wrists; you should transition smoothly using the lower body to start down.
I feel you can still float load the wrists at the top while setting the wrists earlier in the backswing. You should experiment to find what feels and works best for you.
If your grip pressure is about a 3 on a scale from 1-10, the club head will eventually pass “Top Dead Center”, fall on its own and automatically hinge your wrists correctly. What it looks like, is a flat to bowed out front wrist (a la Paul Azinger, Dustin Johnson, Trevino, or Hogan’s supination just before impact). Now we’re talking. This wrist hinge has the club face square to the plane. You will know it is right if the toe of the club looks about 45 degrees to the plane not in line with it. This correct passive wrist hinge ALLOWS THE BODY TO TURN AS HARD, FAST OR SLOW THROUGH TO THE FINISH without any need for manipulation. There is no need to stop and throw the club head at the ball. In fact that messes everything up.
In a word, if there's no "give" at the top of your backswing, you start losing your wrist cock almost immediately. If you have some "give," the club doesn't react immediately. Instead, it absorbs the energy at the change of direction and holds it for a few microseconds while you start down, then gives you a bounce that adds power to your downswing. That "give" comes from several areas, like your overall flexibility, but there are primarily two sources that concern us.