What's the idea of installing e2 on pendrive?

I mean,
Is it possible to leave the official fw on dom and install e2 on pendrive?
Does it install a bootloader that will let me choose what fw I want to load?

I m asking this coz before using azup, you must first place the pendrive on the box and do an update..

I don't think this would leave the dom untouched, with the official fw on it...
Am I right?