I don't recall exactly, but iirc it's similar to the rage glitch in hgss.
Beldum gets Dynamic Punch, which coupled with it's ability to use Gravity, is reason enough to get it. Also, realistically speaking, pretty much everything that gets G3 exclusive moves that are useful on it, or can be used on it have extra appeal points.
Fair enough. Kecleons gets Body Slam, which was the crux of the Choice Band set, although you unfortunately miss out on Recover :/