They use the Prolaser III.
ProLaser III Operator's Manual
The technology used by the ProLaser III to measure
distances and speeds is referred to as lidar, which stands
for
light detection and ranging. When the trigger is pulled,
the ProLaser III sends out hundreds of invisible infrared
laser light pulses per second. As each pulse is
transmitted, a timer is started, and when the energy of a
laser pulse is reflected from a target and received by the
ProLaser III, the timer is stopped. From the elapsed time
taken for the laser pulse to strike and return from the
target, the distance to the object is calculated with the
known speed of light through the atmosphere. If the
target is moving with respect to the ProLaser III, a
sophisticated algorithm is used to derive the speed of the
target from a successive number of range calculations.
This speed determination is then displayed to the
operator.