Technically nothing; as it's based on Shield remote, there is no sleep/power button on the remote.
I should be clear, it's a Sleep toggle, not Power. It will put the Shield to Sleep.
You can use the remote to invoke the power options menu, by holding the Back button, but that can only be done on the home menu/lean back screen. Toddel wants to be able to to what that offers from any app, rather than having to go back to the Lean Back screen first