OK, assuming you're on AFPS75 ('cos that's what I'm on).
You'll get your lump sum and any amount you have chosen to commute (most people choose to go for maximum commutation for tax benefit). This reduces you IP by about 20%. So, let's say your pension was going to be £20K with a gratuity of £60K. You go for maximum commutation which will reduce your IP to about £16K and increase your lump sum to about £100K.
You will now receive a monthly 'salary' (your reduced pension) of about £1000 (£16K/12 less tax). Every year (April?) the previous September's Retail Price Index (RPI) is applied to your original IP. You will continue to get £16K but your original fund (£20K) increases by RPI. At you 55 point you IP is restored to £20K + the compound interest generated by 17 years of RPI and your pension takes a significant jump (assuming 5%/year this will be up to about £45K). From then onwards, your pension is increased annually in line with the previous September's RPI.
Hope that is of help.
Edited - getsometimein, can I suggest you get some time in?
The chap will be leaving at his 38/16 point so he is entitled to an immediate (far greater) pension that you will get after your 12 years.