Pokémon X and Pokémon Y

I'm aware of the mega stones, but apparently the Japanese version of the official site mentions that there are further requirements than just holding the Mega Stone.
So realistically, my point was that the bracelets with the same DNA symbol on them may be involved in the process.

I can't exactly see these pokemon MegaEvolving in every single battle, there must be a limiting factor to it in my opinion.

Maybe it'll revert back when you switch it out?