Traditionally, ragequitting is not about making your opponent forfeit. It's about forcing a draw when you are losing.
Up to an including Black/White, when battling over wifi on DS, if you turn off the DS during the battle, your opponent gets a connection error and the match is cancelled. People use this to avoid getting penalized for defeats if they are about to lose.
However, do note that in Black/White 2 if you ragequit you are not allowed to battle for another hour or so. You may lose points as well (as if you lost), I'm not sure.
This is different in online play where when you quit you always forfeit the match. I don't know if there are any specific "ragequit" tactics but it is usually due to using frowned-upon moves like Double Team, or putting all Pokemon to sleep, or anything that adds more luck to the game.