Mechanical lockers engage automatically when force is applied to the differential via the pinion and ring gear. While force is not actively being applied, springs in the locker allow the mechanism to disengage. That is to say that when you press on the gas it locks up, when you let off the gas and turn it unlocks.
Mechanical lockers are well know for the popping and clanking noises they make when engaging and disengaging.
Mechanical lockers can be installed in front differentials but you must have lock out hubs and they MUST be unlocked for on-road driving because of steering issues.