This looks like an early attempt at the 4e defender concept.
Interesting houserule, although not my cup of tea.
Here's the ability (if someone else can help) ...
Riposte
While most scouts rely on mobility to overcome their opponents, some focus instead on taking advantage of the openings their foes provide in combat when making their own attacks.
Class: Scout.
Level: 1st.
Replaces: If you select this class feature, you do not gain skirmish or any later improvements to that class feature.
Ability: Any time the scout attacks a foe that has made a melee or ranged weapon attack against her within the past round (whether or not the attack hit you), her attack deals an extra 1d6 points of damage. This extra damage increases by 1d6 for every four levels gained above 1st (2d6 at 5th, 3d6 at 9th, 4d6 at 13th, and 5d6 at 17th).
This extra damage applies only against living creatures that have a discernable anatomy. Undead, constructs, oozes, plants, incorporeal creatures, and creatures immune to extra damage from critical hits are not vulnerable to this additional damage. The scout must be able to see the target well enough to pick out a vital spot and must be able to reach such a spot. She can apply this extra damage to ranged attacks, but only if the target is within 30 feet.
At 3rd level, the scout gains a +1 competence bonus to Armor Class against any creature to which she has dealt riposte damage, as described above. This bonus lasts until the start of her next turn. If she deals riposte damage to more than one creature in a round, she gains this bonus against all of them; it is not limited to a single foe. This bonus improves by 1 for every four levels gained above 3rd (+2 at 7th, +3 at 11th, +4 at 15th, and +5 at 19th level).
The scout loses this ability when wearing medium or heavy armor or when carrying a medium or heavy load.