I'll give it my best shot. Technically the brace height doesn't change. That's a measurement from the string to deepest part of the grip. The only thing that changes is the contact point of the arrow on the rest.
This method is gaining a lot of use in the target venues. Considering Randy's background, and OCD tendencies, I'm guessing that is what he is doing. The theory is that changing the position of the pivot point (arrow contact position on the rest) can influence the effect of naturally occurring torque. By sliding a rest in and out, and shooting the bow normally, and with intentionally induced torque the sweet spot of the pivot can be located. This is the place where the arrow varies from its intended path the least when shot normally, and with intentionally induced torque.
You can see it without shooting. Draw the bow, see where the tip is pointing. Torque the bow in and out and watch the tip. Move the rest, repeat. The tip may move more, or may move less. If it moves less, its an indicator that you are getting closer to the sweet spot. The distance of the OD varies from person to person and bow to bow. Find the sweet spot and minimize the effects of the torque, and the arrow flies where you want it to easier. From shot to shot, archer induced torque, including the effects of fatigue, are minimized. That's the theory. Arrow length are usually left 'normal'. It is also similar to 'node' tuning.
Again, something being used in the target venues where relatively small spots by comparison to critters are being shot at distance for $. For hunting purposes, for most, probably not a huge issue, unless you have severe OCD tendencies.