Spiralball is a pvp game mode with two different versions, depending upon popularity.
Version 1a:
Spiralball is a soccer type game where teams must shield bump the ball into the opponents goal.
Shield bumping other players would be disabled & pvp combat (if implemented) would be up to the developer.
Version 1b:
A much smaller version that is more like air hockey.
An example http://www.youtube.com/watch?v=foQKwjG9W3c
Version 2:
This second version of Spiralball ( and probably the most fun) is taken from Unreal Tournament 2003's Bombing Run and can be explained more here http://www.youtube.com/watch?v=iQJdYJgJGX0
Shieldbumping crap is fun as hell.
And bombing run is a flippin' fun gamemode :P