If the twitch is holding a large portion of their lip it will not hurt them but calm them. You can usually see this calming almost instantly. It works better for some horses than others, and not as effective as tranq but twitches are a good instant, not long-lasting calming tool
But a twitch does have to be applied properly so it doesn’t hurt or harm the horse.
It is believed that a twitch calms the horse by releasing endorphins as pressure is applied, thus reducing stress and pain.