By using Memory Link, you will be able to encounter one of them at Meteor Falls. Depending on if you used White or Black, they will discuss their pursuit of Ideals or Truth as well as asking if you have seen a green-haired friend of his. Afterwards, he/she will engage in a battle using Zekrom/Reshiram (depends on the game linked), the three Unova starters, Volcarona (the Larvesta egg gift), and Zoroark. If you defeat them, they'll compliment you and leave behind a map that'll lead you to an area that has Unova Pokemon.
Ketchum's corollary to Clarke's Third Law: Any sufficiently advanced tactic is indistinguishable from blind luck.
Though this was eventually Jossed by moving to the 6th Gen, this was still a valid Guess, so I don't see why it was removed (potholes added):