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.