Using Metal thumb tacks to trigger events
He uses the thumb tacks to close a simple button circuit. The tracks are soldered to the individual button inputs.

Then he uses a wire connected to the common terminal (likely common ground) to "press" the buttons by sliding the over the tacks.

You can see the tack wiring on imgur about halfway down the page and the way he presses the keys at 6:40 in the video. You can see the black wire between his index and middle fingers.