Green screen over the 'mirror', then have the recorded running and reaching sequence played on that. Then the camera pans out to show a double's hand match the action of the shot in the mirror - you can notice the different angles of the fingers and the red sweater.
It doesn't prove it's a mirror, but I think he meant that there are two shadows, and one could have been cast due to the light by the stairs reflecting off a mirror.
Sauce is a movie called Contact only know because I just watched it for the first time like two weeks ago
It's a decent movie on the whole about SETI getting contacted by alien lifeforms. Pretty good story but it can get really preachy at times. I'd still recommend it.
Because most people are freaking retards. It's an incredibly clever movie that does what Sci-Fi is meant to do, it makes you as a viewer ask questions the answers to which may alter your views on the world around you.