^ Negative damage causes you to be healed. But if you get hit directly by an explosive projectile (except grenades and cluster grenades), even if you have a vest and the damage is negative/0, you'd still die. The only way to survive is to either dodge or you happen to have a Flame God (the 10 second invulnerability, and in this case you won't die no matter what, but have only 1 hp) or you're god mode due to scripts.
Basically, you get healed to death.