"The two-stage intake manifold is an interesting design that allows the the engine to tune the intake stream to suit the particular conditions of the moment. The manifold itself consists of two sets of runners: longer, small-bore primaries (higher velocity, lower volume), and short, fat secondaries (lower velocity, higher volume).

Manifold Tuning Valves (MTVs) located in the secondary runners are controlled by the PCM in response to engine speed and throttle position. The secondaries are normally closed except at wide open throttle (WOT), under the following circumstances:
The MTVs open at low revs (below 3200 rpm) to improve cylinder filling when intake vacuum pressure/velocity is weak. This provides good throttle response and makes the car feel more torquey.
Between 3200 and 5000 rpm, the secondaries are closed, forcing air through the longer primary runners only. This improves air velocity once the intake vacuum is stronger, maintaining the faster cylinder fill rate. Longer runners help improve midrange torque under most driving conditions.
Above 5000 rpm, the secondaries open again to provide the largest possible volume of intake air. Velocity is maintained by the high-rev demand load.

In addition to the simple mechanics of providing additional air volume when needed, the active manifold helps to tune the intake pressure waves in the same way a header is tuned to the pressure waves of the exhaust stream. Having two different types of runners helps broaden the range over which the optimum pressure tuning is applied. This allows for good midrange torque without the corresponding loss of high-end horsepower found in a single-runner design.