Possible to animate a clip mask?

I would like to recreate this animated mouse over effect:  http://tympanus.net/Tutorials/CircleHoverEffects/index2.html

I've tried animating a mask which works, however I'm not seeing a way to make the mask transparent so it conceals the shape from the outside of the shape motioning inward as you see in the effect. Or is there a way to make a compound shape (as in Illustrator) do then animate the inner points of the shape?