This is called Deceleration (or Direct) Fuel Cut Off (DFCO). When the engine is turning at > 1500 RPM, and your foot is off the accelerator, fuel is stopped to

This is called Deceleration (or Direct) Fuel Cut Off (DFCO). When the engine is turning at > 1500 RPM, and your foot is off the accelerator, fuel is stopped to the engine. This shifts the engine from combustion, where gas explosions cause motion, to motion causing the engine to turn. Because the engine is still in motion, when the RPM drops below 1500, then engine can resume fuel and begin combustion again. When you disengage the clutch, the engine is disconnected from the wheels, and requires combustion again to keep running.