Sorry, my poor assumptioin ...
I don't have the subscritption so I have not played with these features.
Have you checked out the help pages:
From my quick read, it looks like you could place VAR increment layers in certain areas of the face and then use the value of the related variables to set the transparency.
So, Transparency is say ((#VAR_1#%2)*100) and every time the user presses the increment area the transparency toggles from 0 to 100
I hope that helps