Combat Reflexes is one of Fallout GURPS advantages. See: Fallout GURPS.
You have extraordinary reactions and are very rarely surprised for more than a moment. You get a +1 to any Active Defense in combat. You never freeze up in a surprise situation, and you a +6 on any IQ roll to wake up or to recover from surprise or a mental stun.”— Steve Jackson Games GURPS sources
With the abandonment of GURPS, Combat Reflexes was recycled into generic combat perks.