Paralysis, brought to a atop is what I have always heard in relation to this phrase. Literally the horse would stop if the reins did happen to fall and get wrapped around the axle. I imagine it would be a bad accident for all involved. I wonder if this also happens when in horse racing, pacers break their stride or gait and wind up out of the race. I have also heard it used in the context if someone says this phrase about themselves they are about to scream.