I'd like to learn how to make a before-after slider

I'd like to learn how to make a before-after slider

If anyone here knows how/where, could you point me to a tutorial on this please
here is kind what I am trying to achieve
http://www.manpower.com/ on the main page when you hover on one of the face the images expands one side or the other
 and this:
http://www.youtube.com/watch?v=970_2Vbi37E

currently the tutorials i have found just deal with the older sliders that a series of images along.
thx
D