Okay, here's the scoop: the adjustable feature adds two snap tabs at the base of the carrier. Snap these two tabs together, and the bottom of the carrier is cinched smaller, allowing for tiny babies to comfortably have their legs out of the carrier and still be properly positioned. (Some babies just hate having their legs "froggied" inside the carrier, so this works out great for them!) With the snap tabs, the carrier has two positions: fully-cinched or fully open. Many people are completely happy with just these two "stations" of adjustability. The optional snap tab extender is available for those who would like incremental adjustability. Add the snap tab extender, and you're able to *partially* cinch the carrier in about 1" increments between all-open and all-cinched. If this makes no sense whatsoever, feel free to email and I can try to explain it better!