gills are designed to flow one way, not 2 ways. It may not kill the fish or anything like that, but if you're trying to release it as unharmed as possible you want to just move it forwards, or even in a circle. fish build up lactic acid when they're stressed, and too much will kill them. Ever catch a fish, release it and it swims away strong, then 5 minutes later it's belly up? Could take minutes, could take days to die.
so especially after a good fight, or if the fish is out of the water for a while, (it's usually one of these 2 reasons why a fish won't swim away) you don't want to stress it any more than that. Most fish CAN swim backwards, but it's not something you'll see often. Usually in a pinch, if it is in a hole or something like that and can't spin around.
yeah but WHY is it harmful? like what specifically happens when water flows backwards over the gills?