[ChrisNZ]

6-B-1-D says the exclusive steady effort to straighten the bent right arm stretches but doesn't move the left arm. So when a Hitter begins to move the left arm with triceps acceleration, does Extensor Action end, and if it doesn't, is there now a dual force that both stretches and moves the left arm?

I'm no expert but it seems to me to be a question of the direction in which the right tricep is exerting force. For extensor action - down the line of the left arm, for hitting muscular thrust: down , out and forward on plane. So I think is just a case of the direction the thrust of the right tricep is in - if (for simplicity's sake) extensor action was down V and hitters thrust was forward > then hitting with extensor action is both down and forward (for which there is no key on my keyboard!) - hitting without extensor action is only forward.

Of course there is also the accelerating, non-accelerating issue - but I think this is taken care of by the fact that the left arm is tethered to the left shoulder. Theoretically, one could attempt to apply as much accelerating force as one wanted down the line of the left arm - it won't accelerate, until the point when the force gets great enough to rip the arm off!

Chris