The IMR is the basis of Ting jin as far as im concerned. It is basically the idea that a thought will manifest in an unconscious movement.
You can do this for yourself - get a pendulum (anything that has some inertia to it when hanging). Hold it in your hand so it draps over the side of your fist - then concentrate on the end of the pendulum and "will" it to move forward and back, then left and right, then will it to stop, circle clockwise and anti-clockwise. The will manifests in unconscious physical responses.
Now - a Mentalist develops his sensitivity to these movements in order that he can appear to read someone's mind. For instance I can tell which corner of a room someone is thinking of by touching their hand and getting them to think of it.
How is it useful to taiji? The theory (and my experience is consistent with this) is that a conscious movement is preceded by an unconscious one (the IMR), and sufficient listening ability enables someone to detect the unconscious movement before the conscious movement manifests. This is basically Ting jin in pushing hands imho- you can do it already, it's just been developed unconsciously.
It is a hard thing to develop conscious ability with - unless you are interested in becoming a Mentalist I wouldnt recommend bothering with it. My experience is that the conscious skill required for 'psychic' feats of magic does not translate to the skill required when pushing hands - both are taking advantage of the IMR, but they are not complementary.
Bob - IMR is instant, it's just the conscious reading or manipulatin of it that is slow (at first)
Still learning about ICS - reading The Western Gate - a great read, thanks for the recommendation.
Experience is something you don't get until just after you need it