From Bulbapedia, the community-driven Pokémon encyclopedia.
Tweaking is a glitch that first appeared in the fourth generation of Pokémon. It is caused by pressing the left and right buttons continuously whilst holding the up button and riding on a bycicle in fourth gear. It is most easily done in Jubilife City. This glitch will throw up invisible walls or cause a section/route to become black, only seeing the shadows of trainers, items etc.
According to some, tweaking may allow players to catch Arceus, Shaymin, and Darkrai, however, this is unconfirmed.