Normally, if you end a turn with x leftover action points, x will be added to your AC until your next turn. With HtH Evade, it will be increased by 2x, and then by (your unarmed skill) / 12, rounded down.
Example: a character with agility 10, unarmed skill 140%, wearing no armor. His base AC (from agility) is 10. If he ends a turn with 3 leftover APs, his AC will increase by 3 (to 13) until his next turn. But if he has the HtH Evade perk, his AC will increase by 6 (2 * 3). On top of that, it will increase by 11 (140 / 12, rounded down), for a total AC of 27.
This only takes effect if both item slots are empty or contain unarmed-class weapons, which include brass/spiked knuckles, boxing gloves, and power fists.