When the hubs are in the auto position they do just that the automatically lock the hubs via some sort of vaccuum technology.
The lock position bypasses that vaccuum technology and locks the hubs in place manually. Either method requires that the ESOF be switched to 4x4.
The lock position is a backup in case the vaccuum method fails. I'm kind of a new Excursion owner but that is how I understand it and to be honest I'm not sure how you know if the hubs locked or not via the automatic method.
I do know that if you switch the hubs to lock you can reach around and try to turn the universal joint and if they are indeed locked they will NOT spin.
As long as the hubs are in the auto position just flipping to 4HI puts you in 4x4. 4LO requires you to stop, be in neutral, break on (I think) and then switch to 4LO. Taking it out of 4LO requires you to be stopped, in neutral, and flip the switch back to 2WD
It is auto 100% by flipping the switch. The locks are for serious 4 wheeling. With autos the wheels have to slip to engage, sometimes too late, the lock makes sure going in you are in 4 wheel. Make sure you turn them once a month, they will stick and you will need big pliers to move them. Otherwise the auto is nice, I use 4wlo to pull my boat out, you need to hit the brake to get it out also.
According to the manual, that little book that came with the new X, , ,
The auto lock works when the wheels can move and allow a lock to take place. IF you are already stuck then the auto very likely will not engage so the manual allows you to go to 4x4 if you can reach the hub.