A swing bout is a short boxing match on a televised boxing program, put on at short notice, usually after the main event ends prematurely. Sometimes a swing bout may come before the main event--when the preliminary bouts end quickly.
The boxers of a swing bout are not sure if they'll even fight on that TV program. They may get only a moment's notice before they have to enter the ring.
A swing bout has a variable number of scheduled rounds, which is chosen at the last-minute to fit the constraints of the time remaining for the television show.