Blink next button of storyline player when the timeline end of the slide.

Guys, I'm very late to the party, but I was looking for a solution to a very similar problem and found this excellent resource. However I can't figure for the life of me what is going on in the file. Can someone explain please how the mask animation is done? I checked all the triggers and animations but the rectangle seems to be static and nothing seems to be related to the path animation that reveals the blue line. Like there was a hidden script window somewhere that I can't find.

And for that matter, how is the signature added? It's not on any layer, neither is it an image or text box.

Noob here. Thanks for any help in advance.