[FMX] question about a cool mask effect

Sorry about the vague title, not really sure what else to name it, lol.

Look at this effect.
http://www.foulds2000.freeserve.co.uk/bushv6.htm
I’ve also seen something simaler on bit-101

Does any one know how to do this? or atleast some info or a tutorial to point me in the right direction…

thanks.