Mike posted a video a few months back demonstrating a "flexible frame". I saw this as a demonstration of connection between two points of contact on the body: an incoming force (i.e a push) and a point of stability (the ground).

The point he was trying to show, I believe, is that there should be a constant, reciprocal balancing of forces between these two points. There should be as little impediment as possible in the medium through which that force travels: the body.

The "purest" connection between these points is, for all intents and purposes, a straight line. Of course, as the force travels through a human body, the actual "line" of force is constantly changing, but conceptually, it's always a straight line.

This is the fundamental condition for "jin". The degree to which these points are connected relates to the purity of Jin. It doesn't matter what art,weapon or form someone is working with: Their degree of mastery of jin depends on how unified these two points of contact are.