I've heard and read a lot of times that when you drive downhill on gear, the engine consumes almost no fuel. In contrast, when you drive down the same hill and press the clutch (or put the car in neutral), the engine consumes normal amount of fuel.
When you leave the car on gear, the pistons are still rotating at about 1-2k RPM. How then is it possible to get so small fuel consumption? What causes the engine to rotate?
On the other hand, why does the car burn so much fuel when put in neutral?