To practice prestidig...prestidigtat... illusions, one must take another in to one's confidence...or.
The assistant makes use of an optic to discretion between light and dark then store and reference the variance for a pass-fail search. The mechanical function is operated by an Arduino Uno and communicated by our resident artificial automation Jacob.
Seeing as it was noted in the comments, the card coincidentally at the bottom is NOT the card Jacob chose, if you look closely. I am no hack...see what I did there?!
4 of spades is the card selected by Jacob | 4 of clubs is on the bottom of the deck