After a normal attack you can expend an additional shot and make a second attack with the other end of the weapon. You must be holding the weapon in two hands to do this.
This second strike has the same damage value as the prmary weapon attack. If the weapon has the notation Second Strike: (butt), then you are using the but end of a pole weapon, for Strength +2 damage.
Second Strike cannot be used along with any other weapon maneouvre or 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 Second Strike.