(Unofficial) FAQ of the GURPS Fora

[vicky_molokh]

Q: How are Blocking Spells used / rolled? What modifiers do they get?

A:

Originally Posted by Kromm

While the effect of a Blocking spell is equivalent to an active defense, the roll involved isn't an active defense roll. Just to start with, the roll is against a full skill, not (skill/2)+3. On the downside, Blocking spells are affected by the penalties for low mana, shock (DX/IQ penalty for injury last turn), spells "on," and a lot of other things that don't affect active defense rolls, and do not get +1 for Combat Reflexes. In addition, they always cost FP (even high skill can't change this), automatically interrupt long castings (no Will-3 roll allowed), and can't be used repeatedly in a turn (even at a penalty). As they have their own, fairly extensive drawbacks like this, making them subject to the standard run of active-defense penalties as well would be excessive. Thus, Blocking spells aren't affected by standard active defense penalties.