We now have the first real info about the Break Evolution mechanic thanks to a new set poster! The cards are printed horizontally and you place them over the Pokemon they are evolving from just like LV.X cards. Thanks goes to Hannes S. and Michael S. for the translations:
Zoroark BREAK – Darkness – HP140
Break Evolution Rule: This Pokemon has the attacks, Abilities, Weakness, Resistance, and Retreat Cost of the Zoroark it Break Evolved from.
[D] Foul Play: Choose 1 of your opponent’s Active Pokemon’s attacks and use it as this attack. (Note that you can barelyyy see this attack on the card above, but it’s there.)
For example, if you evolve Zoroark from Next Destinies into Zoroark BREAK, it gains 40 HP, can use Nasty Plot from its previous Evolution, and gains a cheaper Foul Play attack. It is “boosted” by the BREAK card and goes super saiyan. The BREAK card conveniently fits over the illustration of the old Evolution, allowing you to fully read the new combined card.
It might just be the artwork but it looks like the background of the card features some kind of special holofoil pattern. And as we saw previously, BREAK Pokemon will be gold-colored, as this Zoroark is.
Blue Impact‘s booster pack artwork will depict Mega Mewtwo X and Zoroark BREAK while the Red Flash booster pack will show Mega Mewtwo Y and Chesnaught BREAK. Both BREAK Pokemon are gold-colored.