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One observation that I noticed a while back is that if you push really really hard on the STJ axis on the plantar surface of the foot then, 6-7 times out of 10 it hurts; for the rest it is uncomfortable.
If you move the finger a few mm off the axis and do it again, it does not hurt or is not uncomfortable and certainly feels different to pushing right on the axis.

I running short on explanations as to why ...

Anyone want to postulate a reason for this?

 

Craig:

Manual force applied plantarly on the foot which is directly pointing toward the subtalar joint (STJ) axis will produce 100% compression of soft tissues and no STJ rotational forces (i.e. moments). As soon as the manual force vector is moved a few millimeters either medially or laterally away from the STJ axis, the soft tissues will be compressed less and the STJ moments will increase causing STJ motion in the direction of the STJ moment.

This is exactly how the examiner can determine whether they are on the STJ axis or not. If the examiner's manual force during the exam produces no motion and the STJ is not maximally pronated (and the patient is relaxed), then the area of the plantar foot where the manual force is applied must be under the STJ axis. I call these points on the plantar foot the "points of no rotation" (Kirby KA: Methods for determination of positional variations in the subtalar joint axis. JAPMA, 77: 228-234, 1987).