I cannot understand this position. It is my understanding that the right hand is pushing on the left thumb (PP#1 point) in such a direction (directionally along the longitudinal axis of the left arm) so that it extends the left arm. The left arm is pulled straight, but it pulled straight by a
push-action exerted by the right arm/hand unit.
I also think that it is possible to push a string (which is lying on a tabletop) into a straight line - by using the tip of a finger to push against one end of the string (in a straight line direction towards a distant target point). This explanation is identical to the idea of a horse pushing against a harness in order to pull a cart in a straight line direction.
We're saying the same thing, Jeff: You stretch (pull) the left arm into a straight line by a right triceps push against the left hand thumb (just as if the left arm was merely a piece of string).
Regarding pushing a string into a straight line . . .
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