Gives you bigger and a little bit more frequent critical melee hits. Since critical hits are a multiple of your normal damage, this skill won’t do much good if you haven’t trained your weapon skill and obtained a powerful weapon.

The ingame help reads:

Skill duration: random.
Type of skill : neutral skill.
Affecting stats: Dex.
It uses no endurance points.
While every warrior always attempts to hit his
opponent in a way to cause the most damage, every now
and then, one is able to defy even his own limitations
and strike extremely hard, sometimes many times harder
then normal. A very difficult art to master, is the ability
to increase these number of critical hits, and their magnitude.
A critical hit can many times save the day against a seemingly
unbeatable foe.