The theory is basically that in 40k ftl works by sending the ship through the warp (hell). Humans use a Gellar field to keep a bubble of reality around the ship while in the warp. The theory is this is humanities firstf tll so they don't know they need a Gellar field.
It's fun but other then the ftl is hell doesn't fit at all with the rest of 40k lore.
It was apparently too intense for test audiences, and this was in the pre-DVD-special-features era when no one bothered to keep cut footage. Maybe they cut too much. I watched it recently because I had heard fantastic things and I was just... generally unimpressed. It was an interesting concept that really wasn't very well explored, and the writing was so stiff.
I would love to see some sort of remastered directors cut of that movie. It has definitely started showing its age, but it’s still high up on my list of great movies.