1. Arching the Left Wrist opens the Clubface. So if you set up your Alignments with a Flat Left Wrist, and at Impact you have a Arched Left Wrist, the Clubface will be more open and the Ball will start Right. There are two ways to correct this:
a) set up your Alignments at Impact Fix with an Arched Left Wrist.
b) set up at Fix with a Flat Left Wrist and increase the Roll of your Left Hand/Arm in order to have the Clubface square by Separation.
2. Both Bending and Arching the Left Wrist shorten the radius of the Stroke, and therefore reduce Clubhead speed. The radius is the distance from the Left Shoulder to the Clubhead. For example, if you shorten the radius by an inch because of Bending or Arching your Left Wrist, that is effectively the same as reducing the lengths of your Clubshafts by an inch (i.e. putting a nine iron-length Clubshaft into a seven iron Clubhead). You maximize radius with a straight Left Arm and a Flat Left Wrist.
3. Both Arching and Bending your Left Wrist change the Fulcrum and thus, the Low Point of the Stroke. When your Left Wrist is Flat, the Left Shoulder is both the Fulcrum and Low Point; when your Left Wrist is Arched, it becomes both Fulcrum and Low Point, and causes the Low Point to move forward of the Left Shoulder (a Bent Left Wrist causes the Low Point to move aft of the Left Shoulder).
Note: I should've read CNZ's post before I posted. He pretty much answered the question.
I'm glad you did post. The statment about the left shoulder being both the low point and fulcrum was a bell ringer for me. More work to be done...
I'm glad you did post. The statment about the left shoulder being both the low point and fulcrum was a bell ringer for me. More work to be done...
Remember though that this is when the Left Wrist is Flat. When the Left Wrist Bends or Arches, the Left Wrist becomes both Fulcrum and Low Point.
This whole discussion also highlights the importance of the Stationary Head, because as the Head moves laterally, the Left Shoulder, and thus Low Point move with it. So if your Head at Impact is behind its Impact Fix position, both your Left Shoulder and Low Point will be behind their Impact Fix position (if you have a Flat or Bent Left Wrist). This can cause all sorts of trouble, quite often resulting in a Bent Plane Line which causes divots to go to the left of the Plane Line, and in turn, Steering/"chicken winging".
One way to compensate for a Head and Left Shoulder that are behind their Fix positions at Impact is to Arch the Left Wrist. Again, by Arching the Left Wrist, the Left Wrist becomes the Fulcrum, and the Low Point moves ahead of the Left Shoulder. Hopefully, the new Impact Low Point of the Arched Left Wrist is the same as the old Impact Fix Low Point of the Flat Left Wrist and Left Shoulder.