Interesting topic!
I agree you won't break necks with twisting alone, but a motion that resembles that from "hanging" could break the dens of axis (incidently, that is what hanging does to the poor guys neck), in which case you could talk about breaking necks.
This will damage the spinal cord on a level that is lethal, so even in this case it's not the breaking in itself that kills.
I will, for obvious reasons, not be any more specific or describe actual techniques.
These can be trained, as in kata application drills or so, but shouldn't be used in sparring, except in very controlled versions, is what I think.
Exploring your martial art on this level widens your view and is in that way awarding, but it could hardly be called "self defence"!
As to wether it really works or not; I say we'd better believe it works and handle it with care, rather than finding out for sure!
Keep training and posting!
arnie