What is Contact and Acceptance?

I am no expert, so don't expect a real answer. There are many good books written on this subject that speak to the topic far better than I can. It's a great topic for discussion, especially for lower level riders like myself.

The question was raised when I wrote about taking a lesson bit-less and bareback. While riding, my trainer kept pointing out how much Speedy was moving his mouth. He acted as though he was carrying a bit. He was chewing, gaping his mouth, and sliding his jaw around. 

Without an actual bit in his mouth, it's hard to think that he was resisting the contact. Was he just going through the motions from muscle memory? Or, is this how he thinks and processes? Since my lesson, I've been watching and noticed that he is still moving his jaw and mouth without a bit.

The USDF Member Guide has a glossary of judging terms that I find quite useful. Here are some relevant definitions:

• Contact: The reins are stretched so that they form a straight line, not a loop. "Correct contact" or "acceptance" of contact is determined by the elasticity  of the connection between horse and rider.
• On the Bit: Acceptance of contact (without resistance or evasion) with a stretched top line and with lateral and longitudinal flexion as required. The horse's face line is, as a rule, slightly in front of the vertical.

What I find interesting in these definitions is that they do not refer to a bit, only contact. With the riding halter, the contact is across the nose and not in his mouth. Many of our bridles use nose pressure as part of the bit's function. The Micklem does that, not to the same degree that the riding halter does, but still.

I know there are many people who use bit-less bridles successfully. Given that the USDF does make rule changes, however slowly, I can foresee bit-less bridles being permitted in the show ring someday.

What are your thoughts on contact and being on the bit without a bit?