Flex is a Tag-Word Ability that was introduced in the first Bakugan: Geogan Rising card set, Secrets of the Geogan.
Cards with Flex allow the player to gain an additional effect if they have a card in play that costs 4 or more. This can trigger off Baku-Gear, Evos, Flip Hero, Hero or Geogan cards in play.
There are no released cards with the Flex Tag-Word. It would have been featured in Secrets of the Geogan and GG.