In your flail model there was no resistnace to the arm 'unfolding'. With this model the wrists are a free hinge and their is a farily smooth release of the angle between the arms of the flail. Any bending of the extending arm of the flail after the start of the dowswing is only due to harmonic oscillations from the bending in transition.
However tour pros who look to maximize the efficiency of their swings do not have a smooth release of the angle. They maintain a (relatively) acute angle until very late in the downswing. This requires resisting the forces that would extend the flail arm more/sooner. IMHO this is done per Bertholy. That is how tour pros who do, achieve a VERY late hit and why they have such a significant bend remaining in ther trail arm at impact.
This resistance from the trail hand at the grip upwards (not away from the target) while the club head's momentum carries it down at the other end of the shaft causes/increases the 'lag' bend in the shaft.
Of course this is not a 2D but a 3D oscillation. The 'lead' bend rotates apx 90 deg with the shaft (club head squaring) to impact. In this position what was 'lead' bend is would now be oriented as 'droop' but of course depending on the frequency of oscillation the orientation and magnitude are likely different.
The key IMHO is that resistance to the 'natural' extension of the flail arm. This is the point of maintaining 'Keystone'. The 'lead' bend of the shaft at '6/100s' is a confirmation of that resistance.
Peter
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