One of the best explanations of this technique I've ever seen.
[video=youtube;iDnEkFSMRik]http://www.youtube.com/watch?v=iDnEkFSMRik&feature=player_embedded[/video]
[video=youtube;iDnEkFSMRik]http://www.youtube.com/watch?v=iDnEkFSMRik&feature=player_embedded[/video]