Since Heinrich is immortal, he doesn't die. When he takes too much damage he will fall to his knees and be unable to effectively attack. If he takes further damage while on his knees he will explode into a pile of blood and body parts. After being incapacitated you will be prompted to repeatedly press to revive, but if Gretchen is alive you can hold and when she is next to Heinrich she will sacrifice some of her health to instantly revive him. While on his knees, Heinrich can attack but it is a single, slow overhead swipe of his scythe. It's really only useful if an enemy is grappling Gretchen right infront of Heinrich.