en passe

This thread has been answered in:
http://www.chess.com/forum/view/general/how-can-this-happen
Steven gave a pretty good virtual explanation as to what happens in #4
This thread has been answered in:
http://www.chess.com/forum/view/general/how-can-this-happen
Steven gave a pretty good virtual explanation as to what happens in #4
Can someone give me a quick version of en passe.
I'm not sure which square it must be done on.