I've just seen this thread.. Don't know how I've missed it!!
A nice little hack. It's similar to what's known as "back powering". With this, a powered hub sends power to the host device.
It's against the USB specifications, but if it works.. great!
Some of the Raspberry Pi guys use this to save a connection. There were a few USB hubs available that supported this "out of the box", but they seem to be quite scarce now. It's viewed as a way of potentially bypassing circuit protection. I don't think that this really applies here as the same socket is used for data and "both ways" power.