I've seen back pressure become an issue, a friend of mine got stuck on an rock underwater and he had to keep the truck floored just to keep it running. This would have been mid way up the door on an early Bronco, sure he was running but he no longer had the power to drive himself off the rock that his diff hit. Sure as long as you can continue moving you'll not create that back pressure but don't get stuck, that's when the pressure will cause the engine to create enough bp to stall it.
