Adds a passive damage bonus to your attacks, and works better if you’re large and strong. Train this to a high level once you’re an expert at hitting with your chosen weapon and dodging the enemy.
The ingame help reads:
Skill duration: random. Type of skill : neutral skill. Affecting stats: St/Siz. It uses no endurance points. While size and strength generally makes one a better fighter, it takes some skill in using that extra power to its full effectiveness. Just being big doesn't mean you are a good fighter, in fact many giants have been beaten soundly by dwarves. It's not brute force that wins a fight, but skill, the size and strength only helps a bit.