In Clusterball the player must collect as many balls as they can from the playing field then fly them through a ring in the center of the stage to collect points. Various weapons and power ups are strewn around the stages, enabling players to steal their opponents' balls or invert the controls of the other player, making movement much more difficult. The balls lie on magnetic ramps and the aircraft must slide on these ramps to collect the balls. As the player tows more and more balls, which create a tail behind the aircraft, the aircraft becomes gradually heavier, therefore more vulnerable to enemy attacks.
Source: Wikipedia, "Clusterball", available under the CC-BY-SA License.