People are always asking how does it work, the short answer is it is Rocket Science. The longer technical answer is...

The HangTimer uses a Tri-Axis Accelerometer to detect accelerations that a person experiences when airborne. This is the same technology used by NASA in their spacecraft - so we're not kidding about the Rocket Science.

In the past accelerometers have been large and quite expensive, however with advent of MEMS technology the size and cost of the accelerometer has been reduced so they can find their way into consumer electronics devices. A nice intro to MEMS can be found on Wikipedia: http://en.wikipedia.org/wiki/MEMS.

You can learn more about the features in the user manual (pdf)

FAQ

• Where is the best place to wear it?
  Clipped to a belt loop on in a pocket as close to your waist as possible.
• What's with the hole in the clip?
  In case you buddy tries to cheat by throwing it in the air, strap it to them with the supplied ties
• What kind of batteries does it use?
  Two AAA batteries, we expect them to last a season or more depending on how much you ride
• How big is the HangTimer?
  It weights 2.5 oz and measures 2.5x3"