Each board shows a grid of identical tiles with exactly one that differs in color, shape, or angle. It lights up briefly, covers itself, and waits for you to tap where the different tile was. There's no time limit for the tap; the challenge is how much you caught in that one glance.
The flash shrinks from 1 second to 0.2 seconds, the grid grows from 4×4 to 5×5, and from board six the difference becomes subtler. After each tap the board reappears with the answer marked, and taps right next to it count as one tile off. The recap shows the shortest flash you still caught, and your best on this device is saved as a single number.
When the board covers up, focus moves onto it, so arrow keys pick a tile and Enter confirms. If you switch tabs while a board is lit, that board is shown again when you come back.
Common questions and answers about this topic.
Press start and watch the board. It lights up for a moment and covers itself; tap the tile that was different. Ten boards make one run.
It goes 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.25, and 0.2 seconds. The last five boards also use subtler differences.
You tapped a tile touching the answer, including diagonally. It doesn't count as caught, but it shows you saw roughly where it was.
Only one number: the shortest flash you've caught, kept in this browser. Boards and taps aren't saved or uploaded.
The flash time is the score. If the page is hidden while the board is lit, you didn't really see it, so it lights up again when you're back.