-5 or -10%
This limitation works like Unreliable (p. B116), except that instead of rolling against a fixed activation number to trigger your advantage, you roll against DX, IQ, HT, Will, or Per (choose one when you buy the ability). Things that temporarily modify your score do affect this roll. For a defensive ability, roll each time the defense would mitigate an attack or a hazard – or once per minute, for constant exposure. -10% for a DX, IQ, or HT roll; -5% for a Will or Per roll. A special version of this limitation exists for the defensive traits listed under Force Field (p. 108):
Active Defense: Your ability only protects you against threats you’re aware of – and only if you make a roll to interpose it in time. This roll is at DX/2 + 3, +1 for Combat Reflexes. If you try to use the same ability more than once in a turn, apply a cumulative -4 per attempt after the first. You roll at -4 if stunned, and can’t roll at all in situations where you wouldn’t get an active defense (attack from behind, unconscious, etc.). -40%.

　This limitation is appropriate for any passive defensive ability, such as Damage Resistance, Improved G-Tolerance, Magic Resistance, Pressure Support, Protected Sense, Radiation Tolerance, Resistant, Sealed, Slippery, Temperature Tolerance, or Vacuum Support.

　Your ability only protects you against threats you’re aware of – and only if you make a roll to interpose it in time.
This roll is at DX/2 + 3, +1 for Combat Reflexes.
If you try to use the same ability more than once in a turn, apply a cumulative -4 per attempt after the first.
You roll at -4 if stunned, and can’t roll at all in situations where you wouldn’t get an active defense (attack from behind, unconscious, etc.).

　If your roll fails, all future attempts to activate the ability (until you succeed) cost a flat 1 FP but are at no additional penalty. However, if you drop to 3 FP or less from this, your ability shuts down until all FP are regained.
　Your ability is independent of your actual active defenses.

It does not count as your active defense against an attack; it is activated alongside it, not as an alternative to it.
You may always choose whether to attempt your activation roll before your defense, after it, or not at all.

Under the Hood: Active Defense
　Active Defense is really a special case of Requires (Attribute) Roll (p. 17), which itself is a variant of Unreliable (p. B116) – hence the 1 FP cost to activate your ability after a failure. The -40% cost comes from two places:

　1. Since Requires DX Roll is -10%, but the active defense is based on half of DX, it’s only fair to double the value of the limitation to -20%.

　2. Having to be aware of the attack, conscious, etc. – and being at -4 if stunned – is comparable to Active Only on Psychometry (GURPS Powers, p. 69) or Directional, Front on DR (p. B47), worth an extra -20%.

　The -20% “Independent” variant of Active Only simply eliminates #2.
It would be reasonable to use these components to build similar custom limitations; e.g., a defense that required the user to sense an attack would combine Requires Per Roll(-5%) with having to be aware of the attack (-20%) for a net -25%.