When an airline overbooks a flight, it must find someone to give up a confirmed seat and agree to travel at a later time.
The question is this: will travelers give up their confirmed seats voluntarily? Or will someone have to be bumped from the flight against his or her will?
You might wonder why anyone would agree to a voluntary airline bump. Why should you be inconvenienced because an airline overbooked? The answer lies in the possible compensation that is offered.
Airline employees don't want to bump you and face your righteous indignation. So they are generally pleased when someone agrees to be bumped.
The U.S. Department of Transportation requires airlines to ask for volunteers before they deny boarding to a confirmed passenger. If you want to trade delay for airfare reward, make it clear that you're willing to volunteer and negotiate a settlement. The U.S. DOT does not dictate compensation for a voluntary airline bump. Amounts are left up to the individual airlines and their passengers.