Funny enough, synchro, or at least something similar, has been seen before, and that's in Pokken. Funny enough, it's been stated that when a pokemon goes through synergy burst, the pokemon and trainer become connected, similar to Ash-Greninja
http://imgur.com/YxWbBw0. There is also one thing people haven't thought about with this form, the giant Water Shuriken. When a pokemon is in Synergy Burst they also gain a Burst Attack, a powerful move that is unique to the pokemon using it and they're more powerful versions of moves they can learn, such as Weavile's Sonic Slash being a more powerful Night Slash, or Machamp's Dynamic Fury which of course is more powerful Dynamic Punch. The giant Water Shuriken on Greninja's back has been pointed out to be an important feature in this form. It also seems to function differently from regular Water Shuriken, considering it only fires off once instead 2-5 times, and it deals a lot more damage than normal. So basically the Giant Water Shuriken is Ash-Greninja's Burst Attack. As for how this works in the games, people think it won't happen because then they'd have a lot of work to do with modeling in order have a pokemon look like it's trainer, but take note when a pokemon is in Synergy Burst and can not mega evolve, they only gain slight cosmetic changes, similar to how people say that Greninja doesn't look that much different. So what if the look to Ash-Greninja is anime exclusive? It's still a Synergy Burst and would be in the games, but the fact it resembles it's trainer is only for the anime to make Ash's Greninja special. As for how this would work in battle in the games, there's been hints that pokemon can possibly gain a 5th move. So what if that's how it works? Like in Pokken the pokemon will not only power up, meaning stat boosts, but the pokemon will also gain a 5th special move that is unique to the pokemon, a Burst Attack? So when think of it this way, it all works out nicely and makes sense to why Kukui is studying moves. Let's also not forget that the stones in the bracelets, greatly resemble synergy stones. But that still leaves to question, why would this mechanic be shown in Pokken, a spin off, first instead of Sun & Moon. Well there's also the possibility that Pokemon Team told Bandai about the concept, and they decided to implement it into game and tweek the concept to make the game more interesting and have it fit. So in the real games it would probably be different in some aspects.