Animated movie clip in mask tutorial

in a kirupa tutorial on how to make a mask, i get the mask effect but that tut doesnt explain how to make the animated movie clip that “flashes” behind. How do we do it?
thanks

Which one are you talking about ?
pom 0]

the tutorial is this one: http://www.kirupa.com/developer/flash5/masking.asp
i can make a motion tweening to mask, but it occupies several frames, while in kirupa tutorial has only one frame…