From the introduction:
Think of the fight scene in Balin's Tomb, in the movie The Fellowship of the Ring. "They've got a cave troll." Samwise defends himself with his frying pan. Frodo dodges around columns, just trying to keep himself (and the Ring) out of the cave troll's reach. The great wizard Gandalf is reduced to fending off orcs with his sword. Only the fighters rack up any sort of body count. Later, Merry and Pippin leap onto the cave troll's back, doing little damage with their daggers, but enabling Legolas to shoot an arrow down its throat, killing it.
Each hero contributes according to their own abilities. Some fight, some distract, some protect the McGuffin... Different sorts of conflicts (physical, social, skill challenges) spotlight different heroes, while the others find ways (effective or comedic) to support the spotlighted characters.
The purpose of this system is to encourage a wide variety of actions during a conflict. Characters really shine when they work together while playing to their own strengths. It assumes the "standard" GM-player setup. The examples assume heroic fantasy, but the system is designed for any genre.