Hydrogen Cannon.

[Frank S]

What is that pressure vessel? He seemed to be washing it with a garden hose? What would cause an explosion?

I have no idea what the vessel is but it would seem that he was probably using some volatile solvent in the hose when the critical percentage of oxygen to fuel ratio was achieved, a combination of the proper temperature in the vessel and a spark from static electricity from his cleaning cloth or whatever he was using created a flash-over. The volume of air space in the vessel versus the nozzle effect of the opening accentuated the thrust of the blast I would imagine death could have been instant if not at the very least his arm would have been vaporized and half his head would have had burns more severe than 3rd degree. judging from the force that carried him away, my vote was instant death