heres how it all works. By default, the Xenium OS will ALWAYS boot before it a Bios when you turn on your xbox. The OS is not related to the bios, but when you boot the OS you can then choose to boot a bios. Once you turn off your xbox and then turn it back on the OS will once again appear. For most of us we tend to prefer 1 bios and generally use just that, but having the occaisional ability to boot another is always nice to have at your fingertips. If you goto Settings > Xenium, you will find a feature called Instant On, with this enabled, you would need to goto the launch menu and select a Bios as your default. Then if you turn of your xbox and turn it back on using the power button then the default bios will immediatly load skipping the OS. Now if you ever wanted to boot another bios or just load up the OS, you would use the Eject button to turn on the xbox (just press the eject button when the xbox is off) and the OS will come up. Allowing you to choose another bios or mess around in the OS itself.
Hope this helps,
AmO