You have to time it right, like when the killer is 10-20 feet away and about to swing at you. used the hemo heal on yourself and it will null his hit and keep you at 100% health and also prevent him from downing you. This isn't hard. I've done this trick 3 times and I've used hemo probably less than 20 time total. I'm assuming its a glitch but its not a glitch that needs to be fixed In my opinion. Its just awesome luck to have and use at the right moment. Which makes this game great.
