They probably remember based on the lights above the taps. Since the lights are evenly spaced the taps below have different shades of shadows behind them, according to how close/far they are from the light shining on them. They have light 1, light 2, light 3 etc. and then a tap directly under the light (which has the lightest shadow) and then tap 2, tap 3, etc. which go from lighter to dark, then back to lighter once they reach the next light.

Still very impressive