What you're actually measuring
When the screen flips to green, a small relay race begins. Light hits your retina, a signal travels to the visual cortex, your brain recognises the change and fires a command down to the muscles in your hand, and your finger finally moves. All of that takes about a quarter of a second — and this test measures the whole chain end to end.
Because attention wanders, a single click tells you little; your average across several attempts is the number worth trusting. Being rested and alert shaves milliseconds off, while fatigue and distraction add them back. It's a tiny window into how your nervous system handles the constant gap between seeing something and doing something about it.
