I'm making an educated guess, and I'm probably wrong, but here's my thoughts:

You know how when you have the iT plugged in to the wall, it shows a screen like "...Charging" (actually mine doesn't because it's broken, but that's beside the point)
I believe that the system is actually booted partially into a tiny linux that just controls functions such as charging and booting the tablet.
When unplugged, there is no such activity-- the device is off completely.

Some things to check out would be the initfs discussions around ITT and the threads on NOLO (the device's bootloader, which probably is responsible for the speedup when plugged in to power)

Anyway, I hope this helps somehow