GURPS Fantasy 205 for "Dismemberment".
Presumably means the attack does no damage unless it inflicts more than HP, since HP/2 is what is needed to cripple.
GURPS Thaumatology: Sorcery 17 finally explains and defines
- Your spell either works 100% or it doesn’t work at all. The specifics depend on the ability. Damaging attacks must specify a threshold (usually a major wound); if the damage is enough to cause it, the spell works, but if not, the target takes no injury at all.
Page 24 gives an example using Reverse Missiles, an Affliction of Damage Resistance 7 converted into 2d.