Hard to Kill
You are incredibly difficult to kill. Each level of Hard to Kill gives +1 to HT rolls made for survival at -HP or below, and on any HT roll where failure means instant death (due to heart failure, poison, etc.). If this bonus makes the difference between success and failure, you collapse, apparently dead (or disabled), but come to in the usual amount of time – see Recovering from Unconsciousness. A successful Diagnosis roll (or a Mechanic roll, for machines) reveals the truth.
Example: Bruno has HT 12, 15 HP, and Hard to Kill 4. He takes 45 points of damage, which reduces him to -30 HP. He must make two HT rolls to survive: one at -15 HP, one at -30 HP. He rolls an 11 for the first one, but on the second roll, he gets a 14. This is above his HT (12), but below his modified HT (12 + 4 = 16). He passes out, and his foes leave him for dead. Roughly a day later, he'll regain consciousness - injured, but not dead!
In a realistic campaign, the GM may wish to limit characters to Hard to Kill 1 or 2.