I was wondering this myself. My other bike being and FI twin race bike, I had to dig into it to really begin to understand the tuning aspect of it. The way it worked on my other bike was that the Idle was set to a small degree (about 4.5) that the valve in the throttle body would be open. This provided just enough air to keep the bike idling when the system views the throttle as closed.
Set your idle too low (2-3) and the bike would cough, hack wheeze, stumble and run like crap in low rpm's because it wasn't getting the air it needed. Most people, in response, would set their idle higher(6-7). Its produced a healthier faster idle, but If your idle is too high, the rpms would hang when you closed the throttle from the open position.....because while the throttles supposed to be closed, you still have more air rushing it seemingly from nowhere.
So working that reasoning backward, I figured this could be like an inline valve that works similar to one of those covers you see on the exhaust stacks of a semi....That lays over the top and keeps the rain and birds out, but when the exhaust needs to escape, it pushes the cover open. Perhaps this could be the reverse, when the bike requires more air, the vacuum opens the valve to let more in. The more likely scenario, it probably has a spring loaded plate that you can tweak to a degree to create an intake leak as a complimentary inlet to the airbox/throttle body and smooth out any low chugging coughing caused by the throttle plate being closed to too small a degree at idle because it was set to facilitate the open throttle fuel mixture.
Any changes it would cause at wide open throttle in higher gears where it could run super lean might be negligible probably because the bikes in closed loop at the point and adjusting the other intended intake as it gets readings from the sensors. Making the air intake valve an off the radar, complimentary mini intake that can't be automatically adjusted/stifled by the ECM/ECU. Just my theory.
2012 Highball w/ Power Commander and RPW Big Slash.
2006 Buell Firebolt XB12R Raced mapped/Gutted Exhaust.