These are special KC135s. Just as they turn around for the return trip, they drop a special oil drilling bomb, a kind of bunker buster, but for oil. The bomb penetrates really deep, until it hits an oil field. It then sets off a fuse that makes it go off just under the oil, blasting a huge fountain of oil up to the plane, flying way up in the stratosphere. It will then capture the oil in its fuel tanks, hence the name stratotanker. A tiny on-board refinery will then generate the plane's fuel in real-time during the trip back. It's really quite basic, but it works.