I'd assume that it's engaged with a switch as well. Make no mistake, the locker is for badass off road situations at a slow speed. That's why it is typically used in 4lo on a 4x4. And it is not for snow.
I have a 2011 FX4, for the e locker to work you have to be in 4 wheel drive, it doesn't matter high or low. Then you pull out the button and it applies the e locker to the rear wheels. When you don't need it any more just push the button back in.