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?

Which one are you talking about ?
pom 0]

the tutorial is this one:
i can make a motion tweening to mask, but it occupies several frames, while in kirupa tutorial has only one frame…