basic trolley scenario
The trolley car is running - out of control! - down a track, on which five people are helplessly tied. (According to some versions, they were left there by a Mad Philosopher. Anyhoo, they're there.)
Fortunately, you happen to be standing next to the rail switch that will send the trolley onto another line ... on which, a single person - but only one! - is tied down.
Look at the picture. I think it's clear. (And no, you don't have time to run and untie them all. Or even one. Thanks for asking.)