It could take 2,000 hours to unlock this hidden feature in Pokemon Platinum, HeartGold, and SoulSilver, but at least Professor Oak might tell you you're doing a good job

Professor Oak smiling, raising his hand in the Pokemon anime.

I've spent more hours in the fourth generation Pokemon games than any other game I've ever played, but I can safely say that I've never seen this obscure post-game feature, nor do I think I ever will. 

In Pokemon Platinum, HeartGold and SoulSilver, trainers aspiring to be the very best can take on the Battle Frontier in the post-game, which is full of different facilities to test out your battling skills. One of these facilities is the Battle Hall, which challenges you to build up a win streak using the same species of Pokemon repeatedly, to see how long you can keep winning with just a Pikachu, for example. However, as well as earning Battle Points for prizes, building up a win streak here also earns you fans, including special visitors.