Haha...where did you hear this nonsense from? In judicial
hanging there is a fracture of the second cervival vertebra
(bilateral fractures of the pedicles), tearing of the ligaments and
with distraction (separation) between the the first and second
cervical vertebrae. The cervical spinal cord is severed.
There is no compression of the trachea (windpipe) possible for a
simple reason, the fracture is too high and roughly corresponds to
the level of the larynx, which is above the trachea. In any case,
the cartilaginous rings of the trachea will maintain the patency of
the airway.
How do you know? Have you tried it? 
Twitches are an involuntary response...does not mean there is pain.
Ever tried cutting off the head of a chicken. The body can not only
twitch but sometimes the headless chicken can run around a bit as
well. Judicial hanging is akin to decapitation...the spinal cord is
severed from the brain.
I do not know if it is painful but I have no problem if it
is....just punishment for crime. It's the pain of their victims I
worry about.