You do not tell whether you have fixed BI extents or variable ones. If you have fixed BI extents, truncating the BI does not make much sense. If you have variable extents you might benefit from recovering file system space in truncating the BI.
The -G 0 [seconds to wait] parameter tells the utility not to wait and the truncation is started immediately. AFAIK the default ( 60 seconds for 9.1D IIRC ) is there as some sort of safeguard. When you stop the database and you consider all the caches ( OS, file system, controller, disk ) that play a role in this game you might get the idea that it could take some second until everything is really flushed to disk. At least, that's my understanding.
HTH, RealHeavyDude.