Ces vidéos l´expliquent mieux que moi:
http://www.youtube.com/watch?v=epQusYSLXo8&mode=related&search=
http://www.youtube.com/watch?v=j4r5Kvv4Y0s&mode=related&search=
http://www.youtube.com/watch?v=dasnfFclLW0&mode=related&search=
Convaincu 