1. Chose your sport, delay and sensitivity.
2. Press Simulate Start.
3. When you hear the horn, do your thing as fast as you can and then press Stop to see what time you've achieved.
4. When you hear the signal of departure, move the phone in any direction, to see how fast you react. If you move the phone before the signal you will have a false start detected and shown on screen.
5. Look at your time, and try to beat it by restarting.

The time between the warning and the signal depends on the sport selected. The time itself always have a max coresponding to the sport:

Ice Cross (in between 2 to 5 seconds)
Snowboard Cross (in between 2 to 5 seconds)
Ski Cross (in between 1 to 3 seconds)
Horse Racing (in between 2 to 5 seconds)
Sprint (in between 0.8 to 1.5 second)
Swim (in between 0.5 to 3 second)
Rowing (in between 1.5 to 3 second)
Kayak (in between 1.5 to 3 second)
Motocross (in between 1.5 to 3 second)
BMX (in between 0.1 to 2.7 second)
Speed Skating (in between 0.8 to 1.5 second)