This is a very common one size adjustment system. Many of the popular one size diapers use this system: Bum Genius, Haute Pockets, Knickernappies, Happy Heinys, Gro Baby and Tiny Tush
Mommy's Touch has a snap down system that is unique, but still uses snaps.
To adjust, simply snap the top layer of snaps onto a row of snaps below. It's that simple! Due to the ease of use, the snap down adjustment is great for adjusting between children of different sizes.
Fold over adjustment
The fold over adjustment is found in one size fitteds like Marvels and Mommy's Touch hook and loop option. A row of snaps (or aplix) on the inside of the diaper folds over to adjust the size. It's also a very fast system to adjust. Depending on the diaper, this can be a little bulkier.
The internal leg adjustment is becoming a very popular option lately. Found in diapers like Softbums, Fuzzi Bunz and Rocky Mountain Diapers, the diapers change size through adjusting the leg elastic. This system distributes the fabric more evenly through the diaper, so these styles can have less bulk than snap down diapers. There's often a greater degree of adjustability with the leg elastic, so it gives a more customized fit. Since adjusting the elastic takes more time than using a snap down rise, these diapers are more practical for use on one child as he or she grows rather than switching often between children of different sizes.
Each of these adjustment systems has clear advantages, so it really depends on what's most important to you!