Hello, I'll give my 2 cents. Many of these techniques are going to be happening at the same time, but what distinguishes Foreshortening is the change of shape. I've attached an image here that I'll refer to, to help explain. So if we take the shape of a hand and turn it down to face us that is Foreshortening (see 1). The shape of the hand has changed into something less recognizable and it has gotten much shorter (hence the name Foreshortening). Because the hand is attached to the arm, when we turn the hand to face the viewer it will almost always overlap with the forearm (see 2). But depending on how close we are to the hand (or how close the camera is), we may not see much or any diminution or convergence. If we tilt the arm down slightly (see 3) you can see this better. Once we get close to the hand (see 4), diminution and convergence come into play. By combining all the techniques we get a much better illusion of depth. So what determines a "change of shape", I think that depends on the frame of reference. So for us humans most things stand tall and straight, so when they tilt towards us they become shorter. Hands are more recognizable when we can see the whole palm and all the fingers. A person is more recognizable standing straight (see 5) as opposed to from a top view. If we were birds the top view would be more recognizable and we might call this technique of objects tilting and changing their shape: forelengthening. I hope this helps.