Gen 6: So the new thing in this one is you can temporarily evolve select Pokemon into a new, far more powerful form with it's own unique design, ability and stat distribution. It lasts until the battle is over but they need a specific held item and you can only mega evolve one per match. Doing so is exceedingly stressful for the Pokemon in question and even painful so only the very best trainers can safely help their mons reach these new powerful heights so they can realize their full potential and successfully commanding your mega evolved mon is the mark of a true bond between trainer and Pokemon.

Gen 7: you give your guy a crystal do a lil dance to sync both of you up a bit and that let's them do one really strong move once.

Gen 8: you can let one Pokemon huff the rift fumes to get mega huge large for a few moments. If you are lucky they get a sightly different design and one unique move. No real bond needed here.

Gen 9: whatever the fuck is going on with crystallization