If you're the type of person who uploads how-to videos to the internet, you better make damn sure that you know what you're doing!
The man in this video is constructing a house or building of some kind and the time has come to add the roof trusses. So he flips on the camera to detail to his viewers exactly how it's done. After a brief description of how he's going to do it, he lifts the truss into place...only to have it immediately fall down, just miss his head and knock over and break his tools. Bummer.
Seriously though, good thing he stepped out of the way, or else that truss would have no doubt chopped his head clean off.