I am not sure as I haven't totally experienced this, but I have read that it is down to the energy paths within the body trying to regulate their usual flow.
Sometimes they can come up against a blockage if we stand or sit in a particular way that restricts their movements, or even if we have been used to restricting their movements and suddenly they can flow more freely causing that involentary jerk.
Sometimes from my own experience I can feel that my posture is bad and can sense a kind of swelling up of energy, but once I correct my posture it seems to go.
I hope this is of some help, maybe someone who knows more on the subject may care to add something or even correct me
May all beings live in peace and harmony,
May all beings live wisely,
May all beings be enlightened.