What does decider do on this card? I don't see any options here. Does it just force you to cast it as soon as you're able??

Nothing at all - It's literally there to count as an extra copy of the keyword. The card operates on a similar basis to Muscle Burst and the like, so having subsequent copies not count the previous ones would be odd.

Its really just to boost the number of cards with Decider since without a certain critical mass of them in the set, theres no incentive to even use any.