I don’t if it’s even physically possible for me to be this relaxed in my wrist.
Any advice on how to practise this?
Bonamassa - You Upset Me Baby - LIVE [1:55]
I can play it at the same speed if I use my elbow but that makes me unable to play anything else after…
Does he use some kind of rotational movement to makes his fingers turn into spaghetti?
It’s not easy if you hit so many strings, cause they produce a lot of resistance, so I’d say most people automatically ‘lock’ the wrist for not loosing the pick.
Basically you already told how it’s done the wrist itself has to be relaxed, the motion then feels like swinging something around.
If you keep relaxed the whole system needs almost no strength at all, keeping relaxed when playing aggresive is something you need to get used to.
On multiple strings it gives me a hard time to, I try angle the pick than in a way that it produces as less resistance as possible.
Just rembered that I saw that explained by Guthrie:
Probably more reliable than my poor description 
“This” could be a lot of different movements - he’s got long sleeves so we can’t really see what he’s doing. I would caution against seeing moving parts and automatically assuming that relaxation has something to do with it. More than likely this is just a specific combination of arm and wrist movements, and doing it properly means copying them / learning them. Strumming is just as complicated as any other picking motion - after all, it’s the same joints, same muscles.
For a good slow-motion look at one type of strumming we have investigated, you can check out Joscho Stephan’s truly masterful chord strumming:
I haven’t worked on this but the hand and arm movements are right there if you’d like to try and copy his exact setup. It would be a fun project.