The theory is it smoothes the airflow. It does have less restriction than the stock down tube (a.k.a. snorkel), since air does not have to travel to the bottom of the air box then back up and through- but I'm with you Ian, I doubt the efficiency is anything like the gains with open pods.
Steven- my experience in slowly chopping away at my air box is that the midrange falls off with the stock needles. However, either shims, TBS needles, Thruxton needles or a combination of the above cures this.
