You can find several hybrid designs on the market, but the most common is a 2.5-inch version meant for laptops, Seagate's Momentus XT SSHD. Seagate refers to the caching logic it uses on the Momentus XT drives as Adaptive Memory technology. The thinner 7mm, 2.5-inch drives that Toshiba and Western Digital recently announced are destined for Ultrabooks. They will likely use similar technologies with similarly suitable names, although either company might opt to skip caching and produce a dual SSD/hard drive in a single physical package. In any case, a caching algorithm will track the files you load the most often (operating system files, applications, and the like), and store them on the SSD portion of the drive. From that point on, these files will load into memory much faster than they did from the mechanical drive, although some overhead will be involved as the computer determines whether the file in question resides on the SSD. No caching will have occurred the first time you use a hybrid drive, so its initial performance will be the same as that of a mechanical hard drive, but the speed will increase over time.
To test a current implementation and to determine how much improvement you can expect over the long term, we ran a special version of WorldBench 7 six times using a 750GB Seagate Momentus XT hybrid drive with its 8GB internal SSD.
Over the course of the six runs, system boot times dropped from 35 seconds to 31 seconds, and the WorldBench 7 score rose from 112 to 116. That's about a 12% improvement in boot times, and a 4% jump in WorldBench. However, the WorldBench 7 score of a nonhybrid, 5400-rpm drive also climbed by 4 percentmost likely due to Windows 7's own caching technologies. The standard drive showed no decrease in boot times, so the current Seagate hybrid drives do offer some benefit.
WorldBench 7 measures application performance, not the load times of the applications themselves, though subjectively the load times seemed only slightly faster after the first pass in my hands-on tests when I eliminated the Windows prefetch and swap file. Let's call that further, marginal evidence that a hybrid can make a positive difference in your everyday computing. Just for comparison's sake, a good SSD scored more than 40 points higher on WorldBench 7 on the same system.