Wrists play an important role in the swing, to be certain. But this is one of those areas, for me, where I can unconsciously start doing the WRONG thing and end up with that slice creeping back into my rounds. Ball flight is always a result of a marriage between club path and club face position at impact. This means it's easy to misdiagnose what's happening to oneself on the course if one suddenly starts hooking or slicing the ball. I've posted before about the various characteristics of different ball flights; it's up to each individual golfer to figure out his or her tendencies in order to diagnose correctly on the course.
So, what's the correct wrist action at the top? First, let's define what's an incorrect wrist action. For the left wrist, an improper wrist cock would be where the knuckles move more backward towards the forearm (in essence, a left wrist hinge). This is what is defined as a cupped or dorsiflexed left wrist, and it's a move that opens the club face. If timing isn't perfect coming back down, the wrist will not fully flatten and the face will likely be open at impact, causing a slice or fade of some kind. It's important to note that this cupped position at the top might be useful for greenside bunker or flop shots, where an open face at impact is desirable. All of this is related to one's grip as well (a stronger grip can be more cupped at the top of the backswing and still allow for a reasonably straight ball flight). When the wrist is too cupped at the top, it opens the club face and lays off the club. At the same time, a left wrist position that is too bowed (like Dustin Johnson uses) could result in a club face that's too shut and could put the club in an across-the-line position at the top (associated with hooking the ball). Good players can get away with either if their timing is good and they can incorporate manipulations to return the club to the correct impact position.
Now the right wrist is really the opposite of the left. An improper right wrist action would have it too straight at the top of the backswing (knuckles moving away from the forearm), which, if you think about it, mirrors what the left wrist does in an improper action. And for both, there are variations in between. Much has been written and taught about the wrists in golf, and wrists actions among golfers will vary by individual. Some professional golfers, like Tom Watson and Shawn Clement, teach that you should have a slightly cupped left wrist at the top to get the maximum wrist load, wrist cock, or radial deviation (Watson says: 'with as strong a left hand grip as you can use without hooking the ball'). Others, like Hank Haney, teach that the left wrist should be flat at the top (like it should be a the bottom), which means the right wrist would be hinged backward more. The right wrist associated with a left wrist that's too bowed would have the right palm pointing to the sky at the top (like holding a tray or pizza). A flatter left wrist at the top is associated with a more neutral grip. In reality, the right wrist should be more dorsiflexed than the left; this would be kind of like holding a slanted tray--where anything on it would slide off onto the ground.
When I make a club face error these days, it's usually because of a left wrist that's too cupped at the top or not rotating or bowing it fast enough coming down; when I fail to lead with the left wrist, it leaves my club face too open. My outside-in path error is usually an incomplete shoulder turn.
Here's what I've found is a good wrist and grip drill to teach the correct wrist positions at the top. I take my address position with a club and then raise my body into an upright position. From there, I simply move the club straight up and down in front of me using only my wrists. The wrists should cock upwards and downwards, raising and lowering the club; Shawn Clement describes this as hinging on the 'anatomical snuff boxes.' Using the same drill, cupping the left wrist would result in the club moving left of one's head, and too bowed a left wrist would move it right of one's head. The proper movement using this drill would have the club moving upwards right between my eyes. The club face should return to the same position when lowering the club back down using the wrists; if it doesn't…the grip is either too strong or too weak. Once the feel of this is ingrained, it's simply a matter of keeping the arms in front while going to the top, turning the shoulders fully, and hinging the wrists along the same axis practiced in the drill. As with all things in golf, too much or too little can be a very small standard deviation: Too much cup or bow in the left wrist will lay off the club too much or put the club over the line too much (respectively) and will contribute to a slice or hook (respectively), if not corrected coming back down using some artificial manipulation.