the main purpose of an override is so that you can shift while youre at wide open throttle.
there are two main types of overrides..
1. regular override - mainly used in drag racing. upshifting while under power with no clutch. you must come to a complete stop before shifting back down through the gears. if you dont, you risk bending shift forks and grenading the transmission.
2. duneable override - can be up shifted or down shifted without the use of the clutch. you dont have to stop all the way to downshift. you can run most of them just like a regular transmission, however over time they will eventually wear on the shift forks.
most of the time one or the other is used with the use of a lockup(keeps the clutch from slipping). if you dont use an override with a lockup, its nearly impossible to shift the bike unless you let off the throttle and pull the clutch in all the way.