Which happens first?

Okay, just straightforward interest. I have seen lots of videos of "funny" fails et cetera, where people do something stupid and then suddenly hit their head/neck and fall limb. I am always so anxious, since I think that they might have broke their neck, yet I have no anatomical understandment of how easy or hard it is to damage your cervical bones.

So what usually happens - do people pass out of being knocked unconcious, or do they really damage their cervical vertebrae which also damages the spinal nerve/brain? What even means passing out? What is the biological mechanism behind it?

An odd question, which keeps me annoyed. Thank you all in advance!