If both item slots are empty, each unused action point gives you a +2 instead of +1 towards your Armor Class at the end of your turn, plus 1/12 of your unarmed skill.”— Fallout 2 (patched) In-game description
While it states that both item slots must be empty, that is not entirely accurate. It really requires not holding a gun or melee weapon (anything that is visible in the character's hands). This perk will work if wielding unarmed weapons (such as brass knuckles, boxing gloves, power fist), throwables (grenades, rocks), or any non-weapon item.
HtH Evade shares the Vault Boy image with Artful Dodger.