That's a runaround. It's purely an artifact of the turn system that the attacker (the person evading and striking from behind someone) reached his location as part of an active defense rather using the movement allowed by a maneuver selected an instant later. If the defender (the person struck from behind) ends up parrying at -2, that represents him engaging the attacker's fist/foot/weapon/drooling maw
as the attacker tumbles over/under him . . . because in a discrete turn system, the attacker was prepping that attack even as he defended.
Now if the defender fails to turn to face his foe after being attacked, defending, and then getting his own maneuver with which to respond, then any attack on the attacker's
next turn
would not be a runaround but a true surprise from behind. In the space of one second (rather than some arbitrarily small tick between two different people taking their turns), the attacker could've gone anywhere or readied anything, and the defender can't seen him prepping his attack in a way that would allow a defense.
A good general guideline is that if
A goes from where
B can see him to where
B cannot see him
before B has a chance to select a new maneuver, then any attack from
A is a runaround unless
A covered the distance using teleportation or a similar effect. The most likely ways that could happen are:
- On A's turn, A moves around behind B and then A attacks. (The "classic" runaround.)
- On B's turn, A defends against B in such a way that he gets behind B, and then on A's turn, A attacks. (The situation at hand.)
- On B's turn, B attacks A and screws up in such a way that he puts A behind him, and then on A's turn, A attacks. (The "missed slam" case.)
In all cases, the
A ended up behind
B before
B had a full turn to respond, and the relative position really bookmarks movement in progress, not a starting situation. Turn systems are annoying this way.