In a move often seen in action movies (very often!), someone takes the head of another person with both hands, usually from behind, and twists it only once or twice in order to break their neck. A sound of breaking bone is normally played.
It is also claimed to be real by some martial arts teachers:
I'm going to break the neck so once again this is only in a life or death situation and it's very serious; don't play around with this. [...] It's kind of a swift motion; I let go, release the jaw and push and pull back.
- Source: Martial arts lesson transcript
I'm skeptical that this is possible, at least as easy as depicted. The neck bones and muscles should provide quite some resistance. The movie depictions may be largely exaggerated and I think that practically none of the martial arts teachers have had the chance to test if it really works.