Hint #1) One of the places that I have a Tree1.1 is my desk at my 'day job', I have gotten in the habit of leaving the tree plugged into my laptop's docking station. When the PC is docked and powered up (i.e I'm in the office), the USB port is powered and the tree runs. When I undock my PC to go home, the USB port's power is turned off and the tree goes dark.
Hint #2) Now, if you are like me, your desk tends to get crowded with "stuff" and things occasionally get bumped around a little bit. In my case, I wanted to make sure that the tree that I had plugged into the docking station's USB port would keep standing upright. In other words, I wanted to add some weight to the bottom of Tree1.1.
I could have just installed a set of batteries. But what would have happened is that as soon as I undocked my laptop, the USB power would have turned off and Tree1.1 would have switched over to battery power. (If you want to entertain the folks that work the other shifts, then by all means go ahead. :-) But if you really only want have your tree powered up while you are there, then you need another approach.
In my case, I did installed some batteries. (Again, just to weight the tree down and to keep it from getting knocked around my desk.) You could simply use dead batteries. But if you only have good batteries and don't want to drain them, simply turn one battery around and install it backwards. Then the tree's power selection circuit will see 0V for the "battery power".
The advantage of using live batteries, with one installed "backwards", is that you always have a full set of batteries handy... if you happen to want to disconnect the USB port power and switch back to battery power for a portable Tree1.1