the act of actually pushing someone over onto the tracks and killing him, you are actually killing him yourself.

真的动手把人推到轨道上让他死的这种行为，就等于是你亲手杀了他。

耶鲁公开课 - 公正课程节选

Whereas pushing the fat man over is an actual act of murder on your part.

而反之，把胖子推下去则是你自己的直接谋杀行为。

耶鲁公开课 - 公正课程节选