Scratch Ticket

Wel I have seen this effect and couldent seem to find a fla or a tutorial on this effect.

Could someone tell me how to find or do this effect.

More details:

Draging a circle witch will get rid of bit it touches like a sratch ticket.

I know it has something top do with masking.

mask layer which uses drawning API, to create scratch by user?!?! how does that sound, there might not be a tute for exactly what you need, just use your nogging for research.