Plugin Help: Before & After Pic

I wanna add the plugin to one of my existence sites. I want to show before & after image. If you visit the site http://clearcorrect.com/ and go at the very bottom. You will see a before and after images of teeth. If you take the mouse to the left, you will see after image and if you take the mouse to the right, you will see the before image. Do you know what plugin did they use to achieve this result?

Thanks.