Junior manning is 'overtime' to cover unnassigned flying, either voluntary or forced, depending on whether you answer the phone when they call you. It is paid a premium (usually 1.5x). It is based on the seniority system. Most senior eligible is offered the trip (if they make contact with you) and then they work down the list. If no one voluntarily takes the trip by the time they get to the bottom, then they work back up the list. The most 'junior man' that they are able to contact is assigned the trip.

Some people love it, some hate it. If you don't want to work it, don't allow them to contact you.