AFAIK, it is never turned off, at least in the OpenWRT kernel.
It is off until some point in the boot process, so power fed back in that port (for example from a cheap hub) won't power the device on. (but will keep it on once U6 is enabled)
The point in the boot process happens to be just the exact line I outlined above
I will update the Wiki page accordingly, if you don't mind (unless you want to do it by yourself?).
I'll leave that to you. If I have more to contribute, I may do some wiki edits myself.
Ok, I'll do that.
What still puzzles me is the exact role of the 2 Q1/Q2 transistors: my guess is that they are used for power-related things.
I hadn't taken much notice of Q1 and Q2 before, so I just did some research. They are marked "HY4C" and googling indicates it is most likely a general purpose PNP (something like MMBT3906).
Doing some circuit tracing, Q1 and Q2 appear to be used as the power transistors for voltage regulators that regulate the 3.3V supply to other required voltages.
The base connections seem to go back to the Atheros chip, so it presumably handles the regulation but needs external transistors depending on power requirements.
Q1 is for the 2V supply into the ethernet magnetics, and mine measures at about 2.12V out. Q2 is the the RAM power supply, and measures about 2.45V.
I found an exact match for the transistors: these are S8550M power PNP transistors. So you are right: these are just the power transistors for 2 internal voltage regulators that regulate the 3.3V supply down to the 2.5V SDRAM supply and the 2.0V analog Ethernet supply. It looks like we achieved full comprehension in this area now!
In order to obtain full circuit understanding, we are left with the unknown USB power switch U6 chip, and the 3.3V unknown buck regulator with internal Schottky diode U5 chip, I could almost draw the schematic