Flux Capacitors were an item that could only be obtained for a limited time when playing the event Back to the Future. When collecting them, you get an extra 5 seconds added to your timer. (During the event, you had a timer that would kill you if the time ran out).
Besides adding an extra 5 seconds, they can give you exclusive costumes that can't be unlocked in any other way. Collecting 121 Flux capacitors unlocks Dr Emmet's head and body, collecting 200 Flux Capacitors unlocks Biff Tannen's head and body, and collecting 500 Flux Capacitors unlocks Marty McFly's head and body.
- If you played the event Back to the future and you pressed the pause button, it would show you information about the event. It referred to Flux Capacitors as Flux Tokens. This is most likely a mistake, as it is referred to as Flux Capacitors in the appstore and in the Halfbrick Blog.