Often called a Sophies Choice, after that novel and film. The Big Bad has been engaging in kidnapping, and is willing to make a deal. In exchange for a powerful device, a bribe, or an agreement, he'll set one of two hostages free. See, he's got your (crewmate/girlfriend/loving son/mentor/buddy/Sidekick/a Bus Full of Innocents: pick any two), and he's so generous and fair that he'll release one. But he's only going to release one. So, who's more important, and whom are you going to let die? One or both of them will often encourage the hero to choose to save the other.