It is the vacuum shift motor or "center axle disconnect" inside a splined sleeve is resident on the outside shaft (right), when the T case goes 4WD a vacuum switch (valve) draws air from the left chamber, sliding the sleeve left to capture and lock the center shaft to the right wheel shaft, all while the T case is spinning up. At the END of its travel it depresses the switch pin shown left, lighting your 4WD. All in all it is a stupid system (compared to an electric lock) suceptible to leaking (so it doesn't lock) and a great way to bring water from a puddle (if your fittings leak) up into your vacuum pump and crankcase!!