Drive with load (DWL) AKA “target driving”. Put most simply, this technique is accomplished by choosing a “target” rate of fuel consumption and ensuring you don’t fall below it on hills (or in very strong winds, or any conditions which cause load to vary for a given speed). In other words, you will back off the accelerator and lose speed (possibly also downshifting) as you climb, and gain that speed back on the descent. It’s far more efficient than pressing the accelerator more and more to maintain speed on the way up a hill and then releasing it down the other side.