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The continuity equation for incompressible airflow states that the mass flow rate must remain constant along a streamline. This is expressed mathematically as the product of the cross-sectional area of a stream tube and the fluid velocity. Therefore, as the area of the stream tube decreases, the airflow velocity must increase to maintain that constant mass flow rate.

This relationship is a result of the conservation of mass principle: if the same amount of air must pass through a smaller area, then the air must move faster to get through in the same amount of time. Thus, the correct understanding here is that a reduction in stream tube area directly results in an increase in airflow velocity directly related to the need to conserve mass flow under incompressible conditions.