When does the engine provide the greatest braking effect?

The engine provides the greatest braking effect near the governed RPM's because at this range, the engine operates at a high level of efficiency and produces significant resistance against the transmission of power. This is primarily due to the engine's compression and the fact that fuel is still being combusted, albeit at a controlled rate. As the vehicle's speed decreases and the engine reaches this governed RPM range, it creates a strong backpressure, which enhances the engine braking effect.

When an engine is near its governed RPM, it is effectively utilizing its compression to slow down the vehicle. This happens because the engine is still able to generate enough power to resist being turned by the wheels. In contrast, at idle speed or below minimum RPMs, the engine's ability to create momentum and thus brake effectively diminishes significantly because it may not produce enough compression or backpressure to aid in slowing the vehicle down.