GURPSよろず

Back Kick or Back Strike
Hard
Defaults: prerequisite skill-2, or -4 for a kick.
Prerequisite: Karate or any Melee Weapon skill; cannot exceed prerequisite skill.

This technique lets you kick or use a melee weapon against a foe to your rear without making a Wild Swing or changing facing – although your legs must be free in order to change stance. Back Kick defaults to Karate-4. Back Strike defaults to Melee Weapon-2 and you must specialize; e.g., Back Strike (Staff). To throw a “back punch,” use Elbow Strike (p. 71).
To use this technique, you must know that your adversary is behind you! Roll against Back Kick or Back Strike to hit, at an extra -1 if you target a specific hit location.
A Back Kick has standard reach and damage for a kick. A Back Strike can only reach an enemy within one yard, regardless of weapon length. Thrusting attacks do their usual damage; swinging attacks have -2 damage or -1 damage per die, whichever is worse. A Back Strike from a Reversed Grip (pp. 111-112) uses the reach and damage effects of that grip instead of those given here.
In all cases, you’re at -2 to all active defenses until your next turn. This is cumulative with the -2 to parry with a weapon in a Reversed Grip!