Quoted from Manny65:Proximity switch senses the metal ball - Alien uses inductor based sensors that detect the ball through changes in the inductors magnetic field. TZ, STTNG, ToM, Scared Stiff and Road Show all have inductor based proximity switches in a few places. Alternatively you could use a Magnetic Reed Switch to sense the ball - there are aftermarket switch mods to replace standard micro switches with MRS on certain machines, such as the whirlpool on CFTBL and one of the wire forms on The Shadow.
Proximity switches don't have any moving parts like a micro-switch and probably have higher reliability compared to optos. The early proximity switches required adjusting a pot to manually calibrate the switch however modern circuits have auto calibration features that automatically adjust the detector circuit.
Did somebody say MRS!!!! : )