A runaway trolley threatens five people, and a bystander can redirect it toward one. The familiar footbridge variation instead requires pushing a person into its path. Comparing the cases isolates several possible reasons for different judgments: intention, bodily interference, the use of a person, and responsibility for an existing threat.
The switch and footbridge cases also distinguish redirecting an existing danger from making a person part of the rescue mechanism. In the switch case, the five could survive even if the side track were empty. In the footbridge case, the person’s involvement is what stops the trolley. That comparison gives the idea of using someone as a means a concrete role, although it does not establish that this difference alone explains every judgment.