Ok, I can see what he's going for. A needle valve (bottom, right of center?) to vary hydraulic flow and govern resistance instead of loading plates.
You can see Clever Dick holding onto the pressure pipes beneath the 45 elbow. The tank is a just a reservoir.
Looks like he might have used a second chrome steel pipe in addition to the main linear bearing supporting rod and installed a sliding seal inside it to feed the tank. I'd be more inclined to use a fixed seal around a pneumatic strut and let the strut rod plunger the fluid, or a hydraulic cylinder (full size or mini & cammed).
Probably a marketable item to pink gyms where plates are yucky. Chrome rod and a sliding seal aren't cost effective tho and are going to mean maintenance or wet/oily leaks.