This skill lets you do more damage with bludgeons and parry with them more reliably. Note that no level of mastery will let you do great damage with mundane weapons.
The ingame help reads:
Skill duration: random. Type of skill : weapon skill. Affecting stats: Str/Dex. It uses no endurance points. One of the most common and oldest weapons used is the trusty club. Simple, yet effective the club and many other bludgeons are still preferred by man warriors. Clubs, maces, morning stars, hammers, and mauls are only a few of the types of weapons that make up this category. Bludgeons are favored by many due to the fact that even heavy platemail may not reduce the shock of a crushing hit, whereas a edged weapon would glance off harmlessly. Knowing what parts of the target are most vulnerable to crushing attacks is a key in using these weapons.