I think you're referring to shorter grass is more challenged to grow than grass left to grow longer and so the stress on the plant encourages it to produce more....eeep.... would be guessing to say fructans (a type of sugar)....but I fear my a level biology might be letting me down on that one....
is that what you're thinking of? so for people who want to restrict for eg. laminitic ponies, doing it this way can actually be worse due to the way the plant is growing (please correct me anyone if I'm wrong...I fear I might be....)
Shorter grass doesn't grow so well if the weather is hot though - optimum regrowth is only good when graass is no less than 3 inches, shorter than that it cannot cope with the evaporation of moisture so actually doesn't grow so well.
Strip grazing may reduce intake once the grass is short but it often means the horse gets restricted exercise - this then means that the horse doesn't lose the weight in a healthy manner. Track circuit is a great idea.
Generally you need to allow ½ acre of available grazing to allow the horse to exercise enough - this may need to be started by using a lawn mower with a catcher and mowing it short. Strip grazing can often be very hard on the grazing too as the horses are treading on the area repeatedly, damaging the plants and pugging it when it's wet.
Whilst short grass does have more "goodness", a horse grazes below the optimum length. Unlike a lawn for example the grass is grazed lower and doesn't have the chance to recover.
Basically strip grazing is about reducing grass intake. You want to have "manky looking" horse pasture. Do not top or attempt to make it look pretty as this does encourage the grass.