For those on AFPS 75 who won't qualify for the AFPS 15 immediate pension it allows you to sign up for up to four years extra service beyond your 16/38 point so that you can. Will add about £1500 on to your immediate pension, and then at 68 (or what ever the OAP point is by then) about £6000 onto your final pension.
All in all a waste of time. Be surprised if anyone signs up to it.