Skin stretching provides the sensorymotor cortex with new movement options IMHO. There could be some small or maybe not so small local physical effect... after all the PNS ends up, a lot of it, anchored in skin from below, miles and miles and miles of it.. easy to handle. One little stat about that, which unfortunately I can't confirm so far, is that each square centimetre of skin contains 3.54 metres of nerve. There are huge numbers of smooth muscle cells in skin.. It's never seemed much of a stretch for me (pun unintended) to imagine I can have a direct overriding effect on those by just planting my hands on someone, waiting for my fingerprints to stick onto their skin, then widening the distance between my hands slow and gentle, then waiting for up to 2 or 3 minutes, gathering up new slack once in awhile; you can practically feel the brain rushing in to take up residence again as you hold this process in process...but that's probably not even a sliver of what really happens. Given that the nervous system is built in a modular manner with old parts of high specificity as well as new plastic parts (Damasio), it takes time (but not too long) for the information to travel, sort itself and get into all the bits of brain that are relevant. But before long (perhaps 10 minutes) what needs to change, has, and entire body parts may clunk into new configurations while the patient lies there perfectly aware and relaxed and tracking. Bear in mind there has been very little verbal prep beforehand, just that the point of the treatment is to treat the nervous system, and the assurance that they will "feel" stuff, and that it's fine, and that it won't hurt. Frequently patients report that they can feel their limbs "get longer". Which makes me tend to think that there could well be some very funky homuncular shifts happening.
Whatever. Pain decreases. Thinking about all this stuff keeps me very engaged and engrossed in being a practitioner.