so the photographer in the video is doing some sort of collodion process. I'm not all that knowledgeable but I think he's doing a tintype. The tin (or glass if doing a negative) is coated with a collodion material (don't ask me details, that's what google is for) and then sensitised in silver nitrate. The plate must be exposed and then developed while still wet or the sensitivity goes away. The plate is far less sensitive than modern films, thus the head clamp to help the subject stay still for the exposure. This is more or less the process Matthew Brady used for his famous civil war photographs.