121.639 to 121.647 cover fuel. There are no rules on usage, just planning. The redispatch or rerelease works for flag operations because the regs require you leave with enough fuel to get to your destination plus 10% of your enroute time plus to the alternate plus 30 minutes. If you have a 10 hour flight that means you're leaving with an hour and a half of extra fuel. If you can find a redispatch airport at the 7 hour point you can cut the planned fuel by 20 minutes. It could work on a flight from London to New York that the paperwork says you are flying from London to Bangor, Maine with JFK as your alternate and when you coast in the company sends new paperwork that says if you have x amount of fuel you are now released to JFK with Newark as the alternate. Now your 10% many only be 15 minutes and some of the alternate fuel from Bangor to JFK becomes part of the 10% reserve fuel.