In theory, you roll the bakugan on to the cards, whcih have magnets in them that trigger the release. Players use different cards or something to add to their score or take away from the opponants. Or something along those lines. There are a long and complicated set of rules that I suspect most kids ignore!
In reality, you launch them across the room to force them to open or stick them to something metal. DS was also very excited about these and just had to have them for his birthday but they were either discarded or broken within a month.