If you are unarmed, or don't want to attack, you can use any number of physical, non-combat skills (e.g., Acrobatics) in lieu of a combat roll. If you do this, and you win, it's treated as a "Scratch" (no matter how much by). (FUG 44)

If you are using a combat skill and want to fight defensively, you can add +1 to your skill, but if you win the round, you inflict no damage. You can use this to perform maneuvers (e.g., getting away from your opponent), but not to affect your opponent directly.

Thus, if you were unarmed and had an unarmed combat skill, you could use it for a +1.

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So why not get a bonus for the Acrobatics? The distinction here may seem fuzzy, but the idea is that combat skills include the idea of dodging and tumbling and generally being defensive in combat, not just offensive (think of instruction to hit the dirt and tumble and so forth in Basic Training). When learning combat skills, formally or informally, you are learning to do them under fire.

Acrobatics/tumbling/etc. are useful (which is why they can be used here), but are not dedicated to avoidance of being hit, per se. It's to get from point A to point B, and land on one's feet, and so forth. As such, concentrating on it (not attacking your opponent) balances the slightly inappropriateness of the action. [1][2]

Indeed, it's a bit worse than that, since the opponent of a Defensive Fighter, if not faced with a combat skill (against which they have to defend) gets a +1 Superior Weapons bonus. After all, if the other guy is just trying to tumble away, you can concentrate on just shooting him ...

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Skills, Combat Rules, Combat Modifiers