How does what work? Once you are promoted, you are paid based on the new rank you are promoted to. The new rate of pay would commence on the "effective date" of promotion. Mind you, the paper work still has to reach your supporting pay/record support unit for input. Once the input is done, you should start receiving the new rate of pay within 30-60 days.
or elsewhere depending upon the specifics of your inquiry - for instance are you Reg or Reserve? The DAOD (first link provided above), has links listed at the end of it which will get you to specifics.