Description can be found on page 58 of Basic Set: Characters
One thing that GURPS Basic Set p. 419 doesn't make crystal clear, IMO, is how often you roll against HT to not die. For example, if you have HP 10 and take 30 damage, you are at -20 HP. Do you roll once for -10 HP and another time for -20 HP?
Yes, you do.
What makes it clear is the example of Bruno under Hard to Kill (p. B58), who takes 45 damage and passes two death check thresholds, and must roll twice.