[LotFP/LL/S&W/OSR] Knife fighting

Pic from: http://www.gutterfighting.org/

Introduction

In The Brotherhood Of Rose novel by David Morell there was a knife fighting scene between two agents. They used knife hand to attack and off-hand to parry opponent's attack. That is an inspiration for this post. I decided to use LotFP as a default for writing this, but this is easy to adapt to other OSR games like Swords & Wizardry and Labyrinth Lord, as long as you remember that Armor Class values are a bit different in those. LotFP unarmored default is 12, in S&W it is 10. Both of those are ascending. In Labyrinth Lord default unarmored AC is 9 and it is descending.

Mechanics

In this example I use a character with leather armor which is 2 better than unarmored in every edition. In addition the example character has +1 modifier (-1 in descending) to AC from his Dexterity Score.

This is how his AC looks:
Unarmored--with Dexterity--With leather armor--TOTAL
------12------------13--------------------15-----------------15

When using a fighting technique, where you use your main hand to attack with knife AND your off-hand to parry opponents knife strikes, add 2 to armor class. The AC would consist of the following:

Unarmored--Dex--Leather--Technique--TOTAL
------12--------13------15----------17----------17

So, to hit the character and inflict knife's d4 damage to HP opponent needs to roll 17 or higher. Just like by-the-book. But parrying with your hand hurts, am I right?

When the attack roll hits the range of the "technique", which in above example would be 16 and 17 of AC, one point of damage is suffered. Why?

0-12 is the default how hard anything human sized is to hit. 12-13 is how much the guy with DEX bonus dodges and moves around. 14-15 is where the actual armor comes in play. 16 and 17 are your hand blocking the knife's sharp blade, so naturally roll results of 16 and 17, which in AC scale represent your hand result damage.

In A Nut Shell:

Fighting with a knife using off-hand to parry gives +2 to AC, but the AC values in that +2 ranges when rolled with to-hit (with attack bonuses) deal 1 HP damage.