A long time ago, someone with access to the script used it as the basis of a narrative. It was never finished, but it got fairly far along showing you what one possible playthrough would've looked like. Based on that, and some interviews with the developers, the way it worked was that early in the game, the player would have a mishap with the time rift in Sector 14. On occasion, he'd become unstuck in time, to use the B5 phrasing, and end up in some historical situation (the Dilgar War, the Battle of the Line, what have you), and could participate in the action before being yanked back to the present. Depending on how you acquitted yourself in the past, there could be any number of changes when you came back. Broadly, either things would be exactly the same as you left them, different along any number of dimensions (the Dilgar never went extinct, the Minbari War never happened) but still more or less how you left things on the ground, and utterly inimitable to your survival (the Shadows won the war, for instance). If it was the last of these, you'd get blown up fairly quickly and have to reload in the historical segment and try not to break the galaxy this time.