GURPS Init Tool

As per B420:

Whenever you suffer a major wound, and whenever you are struck in the head (skull, face, or eye) or vitals for enough injury to cause a shock penalty (see Shock, p. 419), you must make an immediate HT roll to avoid knockdown and stunning.

Modifiers: -5 for a major wound to the face or vitals (or to the groin, on a humanoid male); -10 for a major wound to the skull or eye; +3 for High Pain Threshold, or -4 for Low Pain Threshold.

HPT does not prevent making K/S rolls for non-major wounds (damage is enough to cause shock, not actually cause it)