When a horse is soft (resistance-free) in the face, you can communicate through his whole body right down to his feet through your reins and seat.
I always strive to “connect their lips to their hindquarters” meaning that when I picked up a rein, I could feel them engage their hindquarters, soften their poll and allow me to connect to all 4 feet.
The horse needs to learn to respond to every nuance of pull, kick, nudge from the rider.
In response, to the degree the rider can feel every softening (lessening of resistance), every try, is the degree the rider is “connected” to the horse. Then, it’s a matter of interpreting the horse’s response to determine if he can handle more pressure, is mad vs confused, hardheaded or a bit slow to process, etc. But the rider has to be able to feel those responses before she can interpret them.
Both of these components comprise what we call “feel”. To some degree, feel is intuitive. Either you have it or you don’t, but it can also be learned.
When we try to teach a horse collection, it is by very tiny, incremental degrees, with the slightest try being rewarded with a release of the pressure we’re applying. It is by that release that they learn, not the application of our aides.
If we miss that small give and fail to release, there’s no incentive for the horse to try next time. They learn to hang on us as we hang on them, both of us getting duller and number (and both probably getting madder and madder!)