[underthetire]

Active probes often times have strain gauges mounted on a beam. Electronic circuit excites the gauges and monitor threshold change to indicate touch. This method is used by Renishaw probes (not sure if all them use it).

Geo

They have both. Strain or LVDT types use less force to measure, and are more accurate, also much more expensive. It's a good probe for CMMs or really high precision equipment.