Almost all shooting chronographs currently marketed to the general public use optical sensors.  The MagnetoSpeed uses electromagnetic sensors (patent pending) to detect the presence of the bullet.  These sensors are immune to muzzle blast, which allows the sensors to be located near the muzzle.  As an added benefit, these sensors are immune to common light problems such as fluorescent lights, low sun, bright sun, etc.

This unique method determines velocity by sensing the difference in time between a slight disturbance in two magnetic fields as the bullet passes by.  This is somewhat similar to the way that an electric guitar senses the vibration of its strings to produce amplified tones.