IÆve been thinking about it last night and drawn up couple diagrams. At least that's how I understand it.
In fact itÆs quite simple. System consists of 2 solenoids, check valve, vacuum reservoir, some hoses and actuator. Vacuum is supplied through suction from intake manifold and kept in check with check valve.
With properly functioning system there are only two modes:
2WD. When electricity is supplied to both solenoids. Right side of actuator is connected to negative pressure right side to barometric pressure. Actuator moves from right to left - free wheeling clutch is disengaged.

4WD. Turn power off and springs return valves to original position. Now left side is equalized with atmospheric pressure and right side is connected to vacuum. Whoosh and free-wheeling clutch is engaged. Even when engine is turned off itÆs still going to lock up. That why when you leave your truck in 4WD for some period there is no waiting time for it to kick in.