So my local store got a book in for display. The employees there got the screen STUCK on it backwards. They tell me that the screen locks on and the button actually actuates the lock via software. So because of this, I'm going to guess that you won't be able to remove the screen when the unit is powered off.
You can detach the keyboard from the tablet while the power is off. You CANNOT detach it if the battery is completely depleted.
So in short, if the Surface Book is 100% turned off, you can hold the "detach" button and the green light will pop up signaling it's ok for it to come apart.
One review said it didn't work unless the battery had at least 10%. That sounds kind of excessive. How much power does it take to activate muscle wire anyway?