The basic idea is that by analogy there are a number of people in a hot air balloon which is losing height and destined to crash, killing everyone on board. It becomes necessary to lessen the weight of the balloon by throwing one of the people out of the basket, thus sacrificing them for the greater good. A debate is held to decide who to throw out. Once one person has been thrown out however, it becomes clear that the balloon is continuing to descend and it becomes necessary to sacrifice out a second person, etc.
There are a few variants on this basic idea. Students can role-play characters (a first person balloon debate), or talk about other characters (a third person balloon debate). Finally, although it's generally in a balloon, it could be used for other things.