Nitrous oxide has two effects. The first major effect is that it decomposes in the cylinder to produce oxygen, thus allowing more complete combustion. The second effect is that the nitrous oxide is usually supplied as a liquid under pressure. When it is injected into the intake manifold, the liquid vaporises and cools, thus cooling the intake air and fuel and effectively increasing its density. Through this second effect, it does increase the amount of air/fuel that the engine takes in -- though this is due to cooling the intake rather than pressurising it. This is not usually classed as forced induction, though.
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