I've heard before of this trick and it works. I know a particular situation in where you should use the Box Trick, it is the following. When your Pokémon reaches level 100, its stats will be final, and they can't be raised anymore. However, if you have ever used rare candies, or put your Pokémon in the day-care where it gained levels, it means that the Pokémon missed out on some stat points. This is because, when you level up your Pokémon with a rare candy, it doesn't gain any stat increases from EV points it has, it only gets the stat increases it would normally get. (If you don't know exactly what EVs are, I suggest you take a look at this page on Serebii:
http://www.serebii.net/games/evs.shtml)
This means that when your Pokémon is level 100, it could still have gotten some stat points. The Box Trick is the solution for that, it is where you put your Pokémon in the box, and take it out again. The stats are recalculated, and it will have gained the stats it has missed out on. I did read that it would only change one stat point at a time, so you will have to do it a few times, if you aren't sure about it. Hope this helps!