Reply to Effect trails?!

I have been struggling to re-create this type of effect when re-mapping my APC40 clip-launch pads to act as a sort of extended Midi Fighter. I'm trying to emulate the transpose effect trail that is demonstrated in this video http://www.youtube.com/watch?v=QrNqF...layer_embedded at 3:24. I do not know how to make a button cause the FX amount to go smoothly from 100% to 0% the way it does in the video. can anyone explain this to me?