Training Science Articles:
Negative Reinforcement is the removal of a stimulus that encourages a behavior to increase in frequency. This stimulus that is removed must be aversive to the learner. If the learner finds it neutral the removal will not be reinforcing. If the learner finds the stimuli appetitive and it is removed this would be negatively punishing not reinforcing.
"Pressure" became a common word to describe "aversive" as that's what is commonly used (physically pushing, space invading, tapping, whipping or pinching), these forms of "Pressure" would be aversive to most learners (though I've met a few who find it like play and enjoy the rough housing). In R- most people use escalating aversives, through some use of pressure.
This doesn't mean all forms of "pressure" are aversive. A massage, a hug, a gentle touch, a good scratch, a kiss, these may be appetitive or neutral to the learner.
Tactile cues are commonly taught to clicker trained horses, this is NOT the same as negative reinforcement (though it can tread the line in some instances, which is why we must be careful). First a positive reinforcement trainer should ensure the tactile cue they wish to use is NOT aversive. They can check this by applying the stimulus and watching the response. If the horse actively works to avoid the stimulus it's aversive. If they ignore the stimulus, it's likely neutral. If they actively seek it, then it's appetitive (for example, when they lean over to let you really reach for a good belly rub). You can also read their expressions to help you deduce how they feel.
Once you know if the tactile cue you wish to use is not aversive, then you teach the behavior without the cue. You can shape, capture, or target/lure the behavior until it is well understood and being offered to you. Once you reach this point you're ready to add the cue. Now you can add your non-aversive, tactile cues. These tactile cues, while they'll be applied to cue and removed when the skill begins, it's not negative reinforcement (as we checked and the stimulus wasn't aversive).
Another key element to tactile cues trained positively is that they are applied once. We don't use our verbal cues going "bbbaaaacckkkk uppp" prolonging the word until you get the behavior, instead we say "back up" and then allow them to do it. The same with tactile cues, a touch on the chest to back up, then stop and let them do it.
Again, it's up to the learner how they feel about different stimuli, be an observant trainer because their feeling are fluid and change moment to moment and in varied situations. Tactile cues are a delicate line to tread, I try to reserve them for when nothing else will do or to mimic a real life situation (like backing away from a stall guard rather than bursting through it, or walking forward when they reach the end of a tie rather than pulling).