What does a hybrid boot do and why wouldn't I always enable it?

Question

As part of Windows 8 there is a Hybrid boot option. What does this do (besides just 'reduce the boot to windows time')?
Why wouldn't I want to enable it all the time; are there downsides?

Answer 1

In internal papers, Hybrid Boot was previously referred to as "Logoff + Hibernate", and it does exactly that.

When Hybrid Boot is enabled, when you shut down the computer, your user applications shut down and your session is terminated, but the OS itself hibernates.

Of course, there are times where you may want to perform a complete shutdown – for example, if you’re opening the system to add or change some hardware. We have an option in the UI to revert back to the Windows 7 shutdown/cold boot behavior, or since that’s likely a fairly infrequent thing, you can use the new /full switch on shutdown.exe. From a cmd prompt, run: shutdown /s /full /t 0 to invoke an immediate full shutdown. Also, choosing Restart from the UI will do a full shutdown, followed by a cold boot. [source]

The above blog post was written while Windows 8 was still a preview. With the general release of Windows 8, the switches on shutdown.exe have changed slightly. To perform a full, normal shutdown, use this command (Or press Shift when you click on Power->Shutdown in the Charms bar):

shutdown /s

To perform a shutdown and prepare for fast startup (hybrid boot, logoff + hibernate), use this command:

shutdown /s /hybrid