Shouldn't be hard for you to find a reference then.You can find various references on the internet regarding ligature neck compression and involuntary convulsions associated to it, written by medical experts who have become specialists in that particular field of research.
It’s doubtful any of us here have experienced it first hand.
Hanging can be painless if done the right way.
Try to keep the knot of the ligature/rope under the chin so that it may compresses the CAROTID ARTERY, JUGULAR VEIN and VERTEBRAL ARTERIES but doesn't blocks the airway (windpipe, through which we breathe). This will ensure breathing as long as one is conscious. As breathing doesn't stop, the pain will reduce considerably as no carbondioxide will built up in the lungs.
Once these arteries and veins are sufficiently compressed blood supply to the brain will stop. Brain will not recieve any oxygen(called Anoxia). Normally 20 to 30 pound (15 kgs at most) of force is sufficient. This will induce unconsciousness very quickly, within 20 seconds or more. Death will follow soon. Due to being in state of unconsciousness its difficult to perceive pain (as in the case of surgery with anaesthesia).
Full hanging is also not necessary to achieve this, partial suspension hanging is sufficient when the victims feets are still touching the ground.
https://www.quora.com/Is-death-by-hanging-painful