Well, basically after you EV train a Pokemon, if you put it in the PC the pokemon's stat would recorrect itself to match the EVs, but I'm not sure if it works anymore in D/P since it doesn't work with level 100's... you could always give it a EV reducing berry in a stat that you didn't EV train so that it recorrects itself.. or so I've heard
So I can't use it on my Event Dragonite?