You're EXP only goes down if you leave a game while it's in the middle. It's called a penalty, and you get them by leaving in the middle of the game, whether you just turn off your Xbox or purposely quit. Other than that, you're EXP does not go down it only goes up. Your rank is based on your EXP and skill level. The more EXP you have, the higher your rank is, but if your skill level is high you get promoted faster. Your skill level is your level, when your skill level goes up I guess you can technically say that you leveled up... but that goes up depending on how well you do in a game and the skill levels of the people you are playing against. If you do bad, your skill level will go down, if you do good it goes up. A fast way to get your skill level up is to play Lone Wolves... basically you have to be one of the top players of the game to level up. That's how it all works