All back emergency!

07:59 Jul 3, 2020 

Just a guess but I'm wondering why there's that chain of commands, with "all" and then "move" as well as "back". Coukd he be commanding the whole crew to rush to the back of the submarine? Maybe that would be enough to tip the nose up and hence stop their downward plunge.

07:26 Jul 3, 2020 

Ah yes, it does!
In that case, I believe it does mean 'all propulsion full astern' — the seaman has just said that the bow and stern 'blades' are stuck in the 'dive' position, so as the sub advances in the water, she is being directed downwards; this is why the Captain first says something like 'stop', and then 'back': with the blades directing downwards (for forward motion), they would act in the reverse sense for backwards motion.

07:11 Jul 3, 2020 

Maybe this is help.
https://www.youtube.com/watch?v=eteXEmhiBIo&t=858s

01:01:33 --> 01:01:50

06:56 Jul 3, 2020 

I would imagine, by opposition with 'forward', this is a commande to once again 'blow' (i.e. force water out of) 'all' of the 'back' (ie. stern end of sub) ballast/buoyancy tanks.
It sounds as if the sub is sinking fast, and they are desperately trying to get her to remount to a safe depth by using compressed air (= light) to blow the water (= heavy) out of the ballast tanks. Normally, a sub is able to float at the surface with the tanks empty (hence acting as buoyancy), and the tanks can be flooded with sea-water to act as ballast so she can dive.
The only other meaning I can imagine would be a command to set all engines to 'full astern' — but I don't know if this wording would be the standard jargon for that in terms of submarinec.