I always think of twitching as a last resort sort of thing. If done properly it should release endorphins to kill pain. done improperly it just causes pain. It is usually done when a painful procedure needs to be done, its used as a restraint and a distraction. There are several different types of twitches the most common is a plyers type. I think Spirithorse is right in saying with the proper training and patience clipping without twitching can be accomplished.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France