How does the sensor work?

One of our readers, Allen, wrote in to ask how the sport kit sensor works. Recently I stumbled across a FAQ by Apple that explains a few things:

How does the sensor know how fast I am going?

A sensitive piezoelectric accelerometer monitors your footstrike when you walk or run and determines the amount of time your foot spent on the ground. This contact time is directly related to your pace.

With my seldom-used electrical engineering background, I have to agree that Apple made a great choice for a sensor for three reasons:

1. Price - Piezoelectric sensors are very inexpensive and simple devices. For example, a basic piezo shock sensor costs less than a dollar from Digi-Key.
2. Battery Life - Piezo sensors can be integrated into a very low-power design. Battery life of the sensor has been a main concern of users. It looks like Apple has done their work and the sensor's battery should outlast five pairs of running shoes.
3. Accuracy - By measuring the pressure of each step, the sensor should be able to measure the approximate pace of the runner. Apple claims that the sensor will be accurate for most users out of the box, with no calibration.
   

Previously many people, myself included, speculated that it might be possible to simply strap the sensor onto the laces of an existing pair of running shoes. This was based on the idea that the sensor would use a motion-based accelerometer to measure steps. However by using a shock-based accelerometer, the sensor must be placed underneath the foot so that it will feel the shock of each step. There have already been reports of hobbyists cutting holes in their existing running shoes for the sensor. So far, it seems that they've succeeded!