The "backup thing" was for Auto Hubs, or PVH Hubs(which were like "auto hubs"), and only to switch from 4WD to RWD.
The Auto Hubs engaged by front axle torque
In RWD the front axle doesn't turn so Auto Hubs stayed Unlocked.
When you change transfer case to 4WD the front axle starts to turn, has torque, so the Auto Hubs would engage, Lock wheel to axle, and you have 4WD in Forward AND Reverse
When you shift transfer case back to RWD, front axle loses torque and Auto hubs "should" Unlock, but sometimes one, or both wouldn't, so front axle would keep turning with the wheel.
Backing up without transfer case torque on the axle would reverse the "drag" torque on the Auto Hub causing it to release/unlock
With Manual hubs you are controlling a gear that connects axle and hub/wheel to lock it, or unlock it, so no "backing up" would be required to Unlock front axle/wheels, a gear is physically moving in side the hub when you turn the Hub from lock to unlock
And just an FYI
In the winter or anytime you might need 4WD, just leave the front hubs Locked all the time, then you can do "shift on the fly"(while moving) to 4high when ever you need to and then back to RWD when you don't need 4WD, no stopping needed.
You do need to stop to get into and out of 4low
2001 and up Rangers have full time LOCKED front hubs, Ford calls it "Live Axle"
Absolutely does not hurt anything to leave front hubs locked, but since you can unlock yours, you can pick up .2-.4MPG while they are unlocked