Depicting the Mega Evolution mechanic that had just been introduced in the Pokmon X and Y video games, these cards were even more powerful than ordinary Pokmon-EX
So, with two Butterfree on the Bench, that's a whopping 40 HP getting healed each turn for all of your Pokemon
There are two main Lost Box variants: SableZard which, as Alex Schemanske (who made top 8 with it in Charlotte) puts it, beats the whole meta apart from Gardevoir and Rapid Strike
All of them have stamps to mark them as Promo cards
Cards without the Symbol are not legal for use in Speed Duel, but Speed Duel cards can be used in the Yu-Gi-Oh
It is meant to weaken, and the method of operation, battle results, and battles damage given to the opponent are basically the same