Is it just me, or doesn't it really have much to do with each other.
If your flat left wrist is perpendicular to the plane at adress, you'll turn it 90* in the backswing and roll it 87* in the downswing until impact, and 3* more till seperation (theoretically). If you have a bowed wrist your clubface will be open and you'll need more roll to square the face.
But you can turn and roll with a cupped, a flat or a bowed wrist, but the rotation will not cause the bowing.
