Clarifying statements regarding the string-ball example.
When I state that an additional force can throw the ball off its orbit, the force doesn't come from the string. The force can occur when the hands do not constantly move along a circular path at a smooth rate of speed - which provides the centripetal force. So, the following factors can disrupt the ball in its orbit.
1) Sudden hand over-acceleration movement (equivalent to Hk's over-acceleration idea).
2) Sudden hand deceleration movement (equivalent to quitting on the shot).
3) Hand motion becomes non-circular (equivalent to HK's steering idea).
4) Sudden change in the radius of the hands' circular motion (which causes a sudden non-circular motion as the hands move to an orbit of different radius).
Also, when I state the clubshaft's flexibility causes the clubhead to move off its orbit, I mean that it allows it. The fundamental cause is the slowing of the hands pre-impact, and the shaft flexibility allows the clubhead to move out of its perfect circular path.
Here is a strobe photo of Bobby Jones swing.
What causes the clubhead arc to be circular? The answer - the circular motion of the hands. Anything that disrupts the smooth circular motion of the hands (over-acceleration, quitting, steering) will disrupt the clubhead's circular motion. An overly flexible clubshaft only makes these problems more apparent.
Jeff.