How it works. For those who care, when the intake valve opens a negative pressure wave travels up the intake until it hits the end of the velocity stack. The negative pressure wave reverses when it hits an expanding cone or opening and becomes a positive pressure wave traveling back down the intake towards the valve. If the positive pressure wave is timed to hit the valve just before it closes, then more mixture gets crammed into the engine, and you get a boost in torque. That's why the intake tract length has to be optimized for the engine rpm where you want the boost and also for the amount of intake duration.