[Varyon]

If you have players declare and roll for active defense even when the foe misses (or have them declare before any dice are rolled), then you should also have them declare and roll for active defense even if an ally negates the attack for you (in this case, for verisimilitude, you'll probably want them to declare before their ally rolls). A character who attempts an active defense loses their Aim bonus, as normal.

Defending even when the foe misses is more realistic than the GURPS default, and is what we see in the above clip (although her "defense" doesn't look like it would have done much in terms of getting her out of the way - that's a flinch response that GURPS doesn't really handle, one that just changes your posture such that you're more likely to get hit in less vital areas; well, actually it's a highly-exaggerated version of that, but it is a cartoon, and such exaggerations are par for the course, there). Of course, it is possible for a skilled combatant to recognize an attack is going to miss, and the further off the mark the attack is, the easier this would be. Offhand, I'd say a if (Attacker's MoF)+2*(Defender's MoS)>5, the defender notices the foe's attack is going to miss and the player can choose not to "use up" a defense (but if the character Retreated, you must use their unmodified MoS above to avoid using up the Retreat).