I'm sure someone else will do better than me, but here is my quick take.

If you have more than a certain number of credit hours in a month you can put the hours above that into a bank. It's somewhere around 80 hours and I think you can save up to 30 hours.

You can then use those hours to pay you back when you have a low month for whatever reason.

You can also buy extra vacation days.

You can also "borrow" from the bank and go negative. Then you automatically pay back the debt if you credit above 80 hours at 5 hours per month.

Biggest reasons to use it are to get up to the GS trigger. Some guys use it to buy an extra week of vacation each year.

One strategy is to fill the bank up on 1st year pay, then pay it out on 2nd year pay. You can also do this if you are upgrading.