a „bostleg“ is the mix between a bootleg and an original sound track.
The participants in a bostleg tournament have to mix/overlay a chosen original extract from a video (which can be a film, cartoon, documentary, video game or whatever) with a new original sound track. See the example below :

Unofficial video from http://deathelectro.com Danger on myspace: http://myspace.com/2emedanger
Clip courtesy of Orion pictures „The Terminator“ (1984)