Gelding scars - the area where the horse was cut when he was gelded - can cause restrictions and gait deviations. Restrictions are usually present if the gelding scar is cool or cold to the touch, indicating a lack of circulation due to scar tissue. Massage can also help to eliminate the restrictions, improving how the horse moves behind.
That's from an article on "Equine Massage Therapy" by Ute Miethe. The article verges on woo continuously, without quite stepping over the boundary. It's full of "I treated this horse and the lame walked" self testimonials, and comments about "releasing metabolic wastes" (rather than "clearing toxins"), before and after photos (where the photos are at slightly different angles, in slightly different light and it's hard to tell if there's really a difference or not, nor how long any difference was maintained) and "it has been proven" without any supporting evidence. If there was any doubt, however, her website shows her true colours. Here she reveals her techniques include shiatsu, accupressure, reiki (how do you "massage" with reiki?) and craniosacral. She also urges regular maintenance, and "less is more" (ie the less you do the more magic it is). She makes comments about diet referring to "the Chinese" (what? all of them?) and "barefoot shoeing" promoter Pete Ramey.
Neoprene is also nasty:
In other cases the signs are more subtle. My horse would always fidget when I put his neoprene galloping boots on. No amount of slapping or yelling made him stop.
It couldn't possibly be because the horse was expecting a good slapping everytime he saw the "special" boots coming out... Could it? Or was he worried about the "special" massage?