From the introduction:
Snowball was originally created as an attempt to emulate backwards-storytelling, starting with the last scene, and moving backwards towards the beginning of the story in discrete intervals (like the movie Memento). However, it has expanded beyond those narrow roots and can easily be used for more “normal” play. Section five gives tips and tricks on how to play Snowball in either mode.
Like its ancestors, Snowball is a role-playing game geared towards a narrative collaboration between a GM and one or more players. Characters in Snowball are not defined by assigning values to a list of pre-approved attributes in an attempt to balance them against an objective standard. Instead, they are defined through a series of Traits, whose only rating is how important such Traits are to the story.
In Snowball, the influence players have on events is not limited solely to dictating the actions of their characters. By invoking Traits, suggesting Ideas, and gambling dice, the players can actively direct the story, essentially assuming the role of the GM for brief periods of time.
Creating a story can sometimes be seen as a hike up the top of a high mountain, trying to reach the peak. In Snowball, everyone starts at the top, and the story naturally rolls down to the bottom, gathering more details in the trip.