Press space, enter, or click the panel the moment it turns green. Pressing while it is still red counts as too soon and restarts that round with a new wait. The run is 5 rounds and your score is the median.
Field test
Your input device is part of the measurement. A mouse click, a screen tap and a spacebar do not all register at the same instant — keyboards and touch screens usually add a few milliseconds of their own. Compare your runs against your own, not against a run taken on different hardware.
Screen readers: this measures a visual reflex. State changes are announced, but announcement latency is far larger than a reaction time, so a run taken by ear will not represent your actual reflexes.
The waits are issued by the server and your rounds are re-checked against them, so a run can’t be faked into ranking.
Clicking before the green counts as “too soon” — that round restarts with a new wait, and you have a limited number of restarts before the run resets.