As the name "In Progress save" implies it saves the game as at the time you save it. Otherwise it would be a "return to start" save and be useless. Anyway in your example you save it at H +45. You come back. You load the saved file (not the original) You are presented with the briefing screen but the time says H+45. You click on "start" and everything is where you left it (and in the same state - damage, ammo, etc.) as the initial save. Hope that helps.