Does licking and chewing mean the horse
has "digested a thought" or has just learnt
something?

This is an idea that has entered horse training dogma relatively
recently.

Our experience with using the positive reinforcement approach to
train shows that licking and chewing behaviour has very little to do
with learning. At the risk of breaking my own mantra of never saying
never, I have to say that we NEVER see this behaviour when
training using positive methods. Yet obviously learning has
occurred, because the horse is able to DO whatever it is we have
trained.

So what does licking and chewing really mean? When does it
occur?

Based on my observations, I feel that the horse is merely trying to
indicate submission. Licking lips and chewing in horses may also
have a similar function to the calming signals dogs use to show
other dogs they are not a threat. Both species are social animals
and have species-specific ways of getting along with each other.

Horses will typically exhibit licking and chewing immediately after
their handler has used some significant degree of physical force to
request the horse to yield to them in some way. It can often be seen
when someone has asked their horse to back up by violently
shaking a lead rope at them, or move their hindquarters over by
quickly swinging a rope while striding toward them, making a horse
run around a pen for longer than they want to and then letting them
rest, and other similar ways of requesting the horse to do
something.

Since we rarely (if ever!) need to use extreme (or even moderate)
levels of pressure in positive training, the horse is not put in a
position where they feel they need to tell you "Yikes!  Yes I'll get out
of your way O mighty one - you up there, me down here!"

The horse may still have learnt to back up (or whatever) in response
to the handler's cue (extreme as it may be), so learning has
occurred, but the licking and chewing is an additional issue.

We do not need to "shout" at horses by using extreme body
movements and escalating levels of pressure - a positive approach
allows light and quiet communication involving subtle body
language right from the start (as does any good horsemanship,
whether positive or not).

Polite and respectful communication does not include one party
feeling a need to indicate their submission. A good leader does not
take their role by force or intimidation.

Having said that, there may still be times when it is necessary to use
whatever pressure it takes to remain safe, or in specific situations,
such as with a pushy, spoilt horse to make a point. However, such
instances are infrequent and not part of everyday training.