Weighted Base
From The Bakugan Wiki
In the Bakugan game, some Bakugan have a Weighted Base. This causes the Bakugan to easily pull off Critical K.O.'s. Also, the Bakugan will bob strangely when rolled. Weighted Bakugan were very rare before Mechtanium Surge, some have only been seen in Battle Packs and BakuSpheres. Heavy Metal (HM) Bakugan are part of Special Attack, and do similar things.
