I don't have cannon backup, but I suspect it's related to something often seen in possessions in fiction; your physical limits are far beyond what you normally make use of.
You have reflex/habit/learned behaviors that limit what you do, pain being the normal warning sign. But if something were to control you and not receive / respond to the limiting triggers, you could perform far beyond your normal abilities. You would damage yourself over time, as those limits are learned for a reason. (Think of what people on PCP are known to do; impressive physical feats, but at great cost.)
'Pushing' yourself in exercise is attempting to expand the range of those abilities; you get the feedback warning of danger/damage, but you ignore it.. a bit. Overdo it, and you hurt yourself. Severe pain can stop you, regardless of you plans.
Neville was given instructions by someone who didn't get the physical/mental feedback, and simply obeyed, having no choice, regardless of feedback from his body. In the short run, it probably did little more than give him muscle aches.. But long term, pushing him 'past his limits' would cause harm.
(A light form of it could make a lovely exercise program, I suppose.)