This module (which has recently been updated) will test your reflexes to sight and sound input. As with most "reflex tests" it is best to perform the measurements several times to gather some statistics.
Many people seem to react more quickly to sound than sight. At first I didn't believe that but after testing myself it seems as if I do!
For the vision test, you need to click the mouse when you see the region turns black.
For the audio test, you need to click the mouse in the "region" when you hear a sound.
You will hear a sound three times and the screen color will change three times during the testing. The order will be random and the time between changes is also random.