Re: Left Wrist. Flat, Bowed or Cupped?
A cupped, bowed, or flat wrist at address, top of the back swing, or at impact are not going to absolutely determine how well you strike the ball. I've seen success with any number of combinations of these. Different people will have different experiences and results based on their own particular swing tendencies and swing styles.
One of the reasons why you see pros with a bowed wrist at impact is because with their higher swing speeds they need to arrive with the club de-lofted as such so they don't hit the ball so high that the wind starts dramatically affecting the shot. A problem for amateurs with slower swing speeds who try and copy this position is that they will have trouble getting clubs like 3-5 irons up in the air. These people would do better from a shot-height standpoint to arrive at impact with a more flat or even slightly cupped position at impact.
Curvature to the ball is more determined by the path of the club coming in to the ball and the blade position at impact. For example, given good contact an out-to-in path with a closed club face will cause a pull hook/draw, an in-to-out path with an open club face would cause a push cut/slice, etc.