[mlangsdorf]

In addition to run-around attacks, you normally can't spend those 5 movement points and make a good attack. Reread the definitions of the Attack maneuver (Basic p 365), the Move and Attack maneuver (Basic pp 365-366), Movement (Basic p 367), and a Step (Basic p 368).

Summary: If Foe A wants to move behind Hero B and attack with a melee weapons, he needs to make a Move and Attack maneuver, and his attack skill penalized and is capped at 9. Even if he gets lucky and hits, Hero B can defend with only a -2 penalty which can be partially offset by Retreating. In the most likely case where Foe A fails to disable Hero B, Hero B on his turn can make an Attack maneuver, turn around to face Foe A (and step back next to him, if necessary because of a Retreat), and attack at full skill. If Hero B makes a successful attack, Foe A can't use his weapon to Parry on defense and can't retreat.

Assuming two equal fighters with ST 13 DX 12 HT 12, Broadsword-16, Shield (Buckler)-16, heavy leather armor (DR 2, 20 lbs), a broadsword (3 lbs), and a small buckler shield (8 lbs), where Foe A is performing Move and Attacks and Hero B is performing Attacks:
* Foe A makes a successful attack 37% of the time, which Hero B retreats and parries 50% of the time - so roughly 1 in 5 attacks connect. 2% of Foe A's attacks are critical hits and Hero B can't defend.
* Hero B makes a successful attack 98% of the time, which Foe A blocks 75% of the time - so roughly 1 in 4 attacks connect. 10% of Hero B's attacks are critical hits and Foe A can't defend.

In GURPS, in a duel, running around behind your foe is not a good idea.