Let's suppose some children were to ram a very large potato into the exhaust pipe of your car. You would probably find it impossible to start the engine because no exhaust gas could escape to atmosphere.. And because the exhaust gas could not get out, little if any mixture could be drawn in. The volumetric efficiency would have reduced to zero. That is an extreme case of course, but it demonstrates the effect
And this is why measuring backpressure is a useful diagnostic tool indeed. In modern cars, the potato is typically a failed and melted catalyst.