The feature is called ‘fast startup mode’ and it is a hybrid between a standard cold boot and restoring your PC from a hibernated state. How does it work? Gabe Aul, director of program management in Windows, explains over on the Building Windows 8 blog:
The key thing to remember though is that in a traditional shutdown, we close all of the user sessions, and in the kernel session we close services and devices to prepare for a complete shutdown.
Now here’s the key difference for Windows 8: as in Windows 7, we close the user sessions, but instead of closing the kernel session, we hibernate it. Compared to a full hibernate, which includes a lot of memory pages in use by apps, session 0 hibernation data is much smaller, which takes substantially less time to write to disk. If you’re not familiar with hibernation, we’re effectively saving the system state and memory contents to a file on disk (hiberfil.sys) and then reading that back in on resume and restoring contents back to memory. Using this technique with boot gives us a significant advantage for boot times, since reading the hiberfile in and reinitializing drivers is much faster on most systems (30-70% faster on most systems we’ve tested).
How much faster is this than a standard cold boot? Take a look at this:
Here’s how fast startup is different to a traditional cold boot:
The speed of the handoff between POST and Windows depends on whether the system has a traditional BIOS or the newer Unified Extensible Firmware Interface (UEFI) … so watch out for the sales pitch for new systems:
One thing you’ll notice in the video was how fast the POST handoff to Windows occurred. Systems that are built using Unified Extensible Firmware Interface (UEFI) are more likely to achieve very fast pre-boot times when compared to those with traditional BIOS. This isn’t because UEFI is inherently faster, but because UEFI writers starting from scratch are more able to optimize their implementation rather than building upon a BIOS implementation that may be many years old. The good news is that most system and motherboard manufacturers have begun to implement UEFI, so these kinds of fast startup times will be more prevalent for new systems.