After a normal attack you can expend an additional shot and make a second attack with the butt of the weapon, for Strength +2 damage. You must be holding the weapon in two hands to do this.
Butt Strike cannot be used along with any other weapon maneouvre or eves special stunt. That is if you did something funny as a part of your attack, it wasn't a normal attack and doesn't qualify for a Butt Strike.