Apparently RBR found yet another tiny loophole in the book (As I'm sure NASCAR teams are looking for those too) where they managed to sneak in a bit of code in the mandatory ECU.
You are allowed to deviate 2% from the standard, and that's what they did. The best trick is of course that it's hard to replicate, and once the season starts, you are no longer allowed to change it so the other teams will have to wait until next year to apply the same trick, by which time it will most likely be banned!