Your Pebble generates a random text string made up of the letters D, U, and S, and you're given 5 seconds to memorize the sequence before it disappears.            

Match the sequence with  a series of hand movements before time runs out -- Palm Down = D, Palm UP = U, and Palm Sideways = S.

Easy.  Then the sequence length increases until it's extremely not!!

Time is running out as you move your wrist to maneuver your pebble piece into a randomly roaming countdown clock to score points.  Advance to the next of 19 levels of increasing complexity, with your player rating improving with each successfully completed level.  You'll have to avoid many hazards as you improve from "One Step Below Terrible" to "Greatest Person in World History".

How fast can you react?  

FlipFast starts with your palm up and held steady.  After a random number of seconds your Pebble vibrates and a timer starts.  Flip your palm down as quickly as you can to stop the timer.  Your result, including reaction time, is shown on the watchface. Can you do better next time? 
 
With 5 selectable reaction speed test levels, anyone can be challenged.