|I haven't seen them in person, just online (from a ME reseller in the UK), but the extenders are basically tabs with snaps on either end that attach to the snaps on the diapers to give you a few more inches. Kind of like a bra strap extender, only with snaps like on the diapers.
OP--My first son was 22 lbs at 6 months, and I wondered if he was going to make it to PTing with the OS, but he did, at 34 lbs. If you need to, the OS can be used with only the last snap fastened one each side (in the non-folded-over setting). I did this briefly and never had any problems with them coming undone.