About reaction time
Human visual reaction time to a simple stimulus like a color change is typically around 200 to 300 milliseconds, with some variation from tiredness, practice and attention. This test measures the time between the box turning green and your click, using the browser's high-resolution timer.
Clicking during the red waiting phase counts as a false start and is not scored, since anticipating rather than reacting is not a genuine measurement. Your best time and recent average are tracked locally in your browser only.