From: Combat Information Center
I think what it means is Save your game at the Endscreen...there's a Save button on the left that will save the postbattle file. If you reopen this "save", you'll go right to that endscreen, before the Promotions and Repair phases. Then "snap" that save, which usually has a Turn Number 1 greater than the actual turn the game ended on. That "snap" will have your force's battle results before any leader promotions or changes to Exp/Mor. I usually make 3 saves for any campaign I play: A "Deployment" one, after my forces are setup but before the Start; a "Progressive" one, where I save every turn or two as the battle progresses, and an "Endgame" one, at the endscreen.
Saving at the endscreen also allows you to "change" the next battle mission if you don't like the one that comes up in the Long Campaign; because you are reloading before the Repair phase, instead of after the Deploy phase, the battle isn't set yet.