Let me preface this by stating once again that I am not a mechanic or and engineer. I take my 2001 Xterra off-road and work on it now and then.
That said, someone asked about how the Auto Hub on a 1st Gen Xterra work on one of the other boards and I answered with;
There is a spring within the internals of the hubs that keeps the engagement teeth of the hub separated allowing the wheel to “Freewheel” when it is not under power from the differential.
The “fixed cam assembly” is “fixed” to the knuckle spindle. It does not turn with the wheel or half-shaft so it is the anchor within the hub enabling the hub to distinguish the half-shaft moving.
When you put the transfer case in 4wd the rotational power, transferred out to the hubs via the half-shaft, causes the “fixed cam assembly” mechanism within the hub assembly (transforming the rotary motion of the half-shaft into linear motion) to push (overpower) the spring, allowing the teeth of the locking the hub to engage.
The “fixed cam assembly” can not “tell” if the wheel is turning (torque from the road pushing the wheel) before this happens because the teeth locking the hub are not engaged until it does (refer to 1st sentence).
They stay engaged until you go from forward to reverse and the “fixed cam assembly” (transforming the rotary motion of the half-shaft into linear motion) releases the spring that then pushes the teeth of the locking the hub to the unlocked position. In 2WD (receiving no rotational power) the spring will cause the teeth to stay disengaged. In 4WD the hub assembly (receiving rotational power) will disengage and re-engage very quickly.
Rocking your auto hub equipped truck back and forth is what usually blows a hub (disengaging and re-engaging while receiving rotational power).
This is why folks change to a Manual Hub; it does not depend on rotational power from the half-shaft, it is always locked or unlocked.
…thought it might be useful to archive it here. Hope it helps someone…