This skill helps you inflict extra damage with a thin blade. Thin blades are light, narrow swords with the weight at the hilt, e.g., a rapier. It is automatically in effect while you are fighting.
This is an important skill for any frontrow guild wishing to use thin blades. Like a dagger, thin blades can be used in a variety of Hand skills, but tends to be less effective at them since thin blades do not have a stabber stat. They do hit harder than daggers however.