Infinite Health Door is a mapping error used to gain potentially infinite amounts of health points. It is found in the level c2a5f, in the chapter of Surface Tension.
To trigger it, the player has to get stuck inside the door, either by getting crushed or standing in the seam. This will cause the health points to climb up as long as the player remains stuck, resulting in potentially infinite health. In a speedrun, this allows the player to perform more damage boosts or simply take loads of damage without consequence for the rest of the game.
¶ Technical explanation
When a door is blocked, it will damage the blocking entity by a certain amount every frame. The damage value on the two doors in c2a5f is -1. Negative damage is not dealt with correctly and will lead to you gaining health instead.
Since the HEV (armor) reduces the damage you take at 5 regular points => 2 HEV and 1 HP and uses rounding for values less than 5, 1 regular point lost with HEV becomes 1 HEV and 0 HP lost. Thus you must have zero HEV otherwise HEV will increase and health will remain the same.
The rate of the damage you receive depends on your framerate, so by changing your FPS you can control how fast you gain health: 20fps is 20hp/s, 1000fps is 1000hp/s and so on.
The health value is a 32bit integer so that means a maximum of 2,147,483,647hp. To reach that value, you'd have to stand in the health door for 24 days (576 hours) at 1000 FPS.
The healing effect can be applied to NPCs as well.
