Per GURPS Martial Arts, the Hook maneuver can be used to pull a foe off balance using a weapon. Combined with a targeted attack to the leg or knee, that gives you the effect you're looking for.
It's not RAW, but it is possible to make a Return Strike (martial arts, p. 78) maneuver with a rigid swinging melee weapon by sacrificing a yard of Reach in order to get closer to your opponent and then performing a sort of "roundhouse" strike so that your weapon strikes your opponent's body from behind.
Here's my revised HR version of the maneuver.
Return Strike
Hard
Defaults: Axe/Mace-2, Broadsword-2, Double Polearm-2, Flail-5, Knife-2, Kusari-5, Polearm-2, Shortsword-2, Staff-2, Two-Handed Axe/Mace-2, Two-Handed Flail-5 or Two-Handed Sword-2.
Prerequisite: Axe/Mace, Broadsword, Double Polearm, Flail, Knife, Kusari, Polearm, Shortsword, Staff, Two-Handed Axe/Mace, Two-Handed Flail or Two-Handed Sword; cannot exceed prerequisite skill.
A Return Strike is a swing with weapon aimed so that it misses the opponent’s body on its outbound arc, so that it strikes the opponent’s back or side on its return. It allows you to strike the enemy from behind while you are in front of them.
To attempt a Return Strike, you must have a weapon capable of making a swinging attack which has sufficient Reach to attack at least a yard beyond your opponent – that is, Reach equal to your range to your foe plus at least 1 yard. For weapons with Reach 1, this means that you must step into Close Combat to use a Return Strike – see
Wrap Shot, below. To use this technique against a large or broad foe, you must have also sufficient Reach to reach over or around them, plus 1 yard.
If you are using a rigid two-handed weapon, such as a Polearm, Staff, or Two-Handed Sword, you must first switch to a Defensive Grip to use this technique.
If you are using a Knife or Shortsword you must be in close combat, but these weapons are short enough that you can make Thrusting attacks with them.
Roll against Return Strike to hit. If the target has never seen this move (GM decides), treat it as an attack from behind. This usually allows no active defense! Otherwise, it’s treated as an attack from the side which gives -2 to most defenses. Damage is unchanged but only the DR on your victim’s back applies.
If you attack with a Return Strike from your opponent’s Side hex, the attack hits the opposite side of their body. Treat this like a regular Side attack, except for the direction from which the damage comes.
If you attack with a Return Strike from your opponent’s rear, the attack hits the front of their body, but they defend against it as if it were hit from one of their Side hexes.
You cannot Retreat or Disengage on the same turn you use a Return Strike. If your opponent Retreats, he gets twice the normal bonus to Active Defenses to avoid your attack.
Wrap Shots: You can combine Return Strike with Close Combat to do a “wrap shot” while in close quarters; see
Using Techniques Together. Treat this as
Striking into Close Combat. If you miss your target with a critical failure, or if your opponent Dodges with a critical success, you may hit yourself! Roll vs. your Return Strike technique. Failure means you hit yourself in the face or head for full damage. Success means that you hit yourself in the face or head for half damage.*
* True story. I have seen SCA fighters "self-kill" themselves when using this technique against the back of an opponent's head. Target ducks, attacker hits themselves in the face with their own weapon.