How does the mirror option "Faces to mirror" work?

Hi,

I can not find the answer anywhere on using the Mirror option "faces to mirror".

Below is a short video what I get. I am doing something wrong? SW documentation didn't help to clarify this.

SW 2014 Sp3

(embeding might not work properly so here is a direct link: http://www.youtube.com/embed/c6uA4NLVssU )

Thank you for your advice

SolidworksParts And Features