The term "engine braking" refers to the braking effect that occurs in gasoline engines when the accelerator pedal is released. This causes fuel injection to cease and the throttle valve to close almost completely, greatly restricting forced airflow from, say e.g., a turbo. The latter of which causes a strong manifold vacuum for which the cylinders have to work against—sapping much of the potential energy out of the system over time, and producing the majority of the engine-braking effect. This vacuum manifold effect can often be amplified by a down-shift, which induces a faster spinning drivechain to engage with the engine.