“The drive will be initially targeted at notebooks, where the drive’s low power consumption will yield the most benefit.
The hybrid hard drive will mount a one-gigabit OneNAND flash chip inside a hard drive, which will serve dual duty both as a write buffer and as a solid-state boot disk for the operating system. The act of writing data to the hard disk, such as user files or cached images while web browsing, would instead be intercepted by the hard disk. Only when the flash memory’s “write buffer” was full would the hard disk be spun up, minimizing the time and power that would be spent keeping the drive’s rotating media spinning. “
So basically the operating system will be read off of the internal flash memory to make boot up from a cold start or hibernation almost instant.
Read more at ExtremeTech