What is he doing.

looks like he is putting the pin back in the boom             welding a bar on the pin so he can drve it in without mushrooming the pin

Pulling the pin  Looks like the threaded rod stripped and he is welding the nut solid.  Acmerod works best and minimum 10 ton power pack

Big question is why hasnt this novice released the hydraulic pressure in the check valves (monoblock) first?

I think he is adding a mount for a "Sidewinder" pump to 4 wheeler missle. For the days when they pull out right in front of you.

He is uninstalling pin

Boom is down and the pin to the right of the welder is already pulled.  There is no danger there but he needs to get something under the cylinder to prevent any burring of the pin or bushing when the pin comes out, unless he's ungodly strong to be able to lift it.  The second section is always a bear.   Once the pins are out getting the cylinder out and down on the ground is always fun.   

My question is why hasn't he taken the keeper off the pin.  That will be a mess if he tries pulling that pin like that

My question, Whats wrong with his left hand. It looks Alien

Wouldnt it be easier to rest boom on ground and do this repair? Seems in the cradle would be the most difficult. 

They are working on the 2nd section ram. Doing it the way suggest would put the work area 20-25 feet off the ground. If it was a 3rd or 4th they would unfold and work close to the ground.