With manual hubs, you turn them to FREE when you're not using 4 wheel drive. That way you don't have to spin the front axle, differential and driveshaft. It creates less drag to save a little fuel and cuts down on noise a bit.

4 wheel drive systems have two controls. First, you have something controlling the transfer case. You shift the transfer case from 2H to 4H, it sends engine power to the front axles via the front driveshaft and front differential. You can also shift to 4L, which reduces your gearing to all axles. On my truck this transfer case is controlled by a floor mounted lever.

Second, the hubs are locked or unlocked. If unlocked, the wheels spin freely from the axles. If locked, they spin with the axles.

The ESOF system combines the two controls into one dash mounted switch.

The ESOF system has two very weak points. First, the automatic hubs aren't real good. They can leak, fail, seize or just be balky. This is why you have a setting to bypass the automatic function of the hubs, just lock the hubs to the axle.

Second, the motor that shifts your transfer case can fail, leaving you unable to shift into 4H or 4L, or worse shift out of 4H or 4L. No one has come up with a solution for this problem, yet.

A third component that often fails is the vacuum solenoid, which is what sends the vacuum signal to the hubs for locking and unlocking.