Reduces the penalty to hit when you’re using multiple weapons.

There’s no ingame information on what the offhand penalty actually is or how this skill works. This skill is essentially the only reference to such a penalty even existing. An old rumor states that it reduces the miss penalty by one per percent, that 40% is enough for any two handed race, and that Cyclopses need not train this skill. I have no idea what the truth is. Using as many weapons as you can hold is basically always worth it though, with or without this skill.

The ingame help reads:

Skill duration: random.
Type of skill : neutral skill.
Affecting stats: Str/Dex.
It uses no endurance points.
While most of us possess two hands, some more than that,
we are usually more skilled at using one than the other.
In many cases, the offhand is only a spectator while the other
hand performs most duties. The same applies to wielding weapons,
our primary hand is much more adept when using weapons. Through
rigorous training, one can become skilled enough to use weapons
in their offhand(s). If you are really good, you can probably
hide that you have an offhand at all.