Tossbag

I'm creating an app that has a form which is larger than the viewable form. I know I can use scroll bars to navigate the form but I saw a cool app (can't remember which) that would pop up a semi opaque(transparent) image on the edge of the form which would allow you to scroll the form by clicking on it.

I am trying to create this function but all my ideas have failed. Can anyone point me in the right direction.

Thank you in advance.