Just to set the record straight, I found this article with words from the BoosterPlug Inventor:
Booster Plug - webBikeWorld
"The idea itself is actually rather simple: if you can trick the computer to think the ambient temperature is lower than the actual temperature reading, it will enrichen the mixture a little which will improve acceleration and throttle response.
And it gets even better: All modern bike have an oxygen sensor in the exhaust to provide a feedback that will adjust mixture back to factory determined level. This actually works to your advantage as there’s a certain delay in this feedback.
This means that as long as you maintain a steady speed (steady throttle, steady RPM), the feedback from the O2 sensor will make the computer adjust the mixture back to the original lean level (good for mileage). This is called “Closed Loop” operation.
As soon as you move to a different cell on the injection map by changing the throttle position or the RPMs, the oxygen sensor feed back cant keep up, and the bike will rely on the basic fuel map and input from temperature/air pressure sensors (like the first generation of fuel injection computers did). Now you will have the slightly richer mixture that is improving the drivability a lot. This is “Open Loop” operation
So you will get the best of two worlds: Improved throttle response where you need it, while maintaining a good mileage – quite brilliant actually!"
I can attest to this theory as I have experience tuning the ECU on my Harley Davidson with a full-access tuner (not a piggy-back style tuner). With live-telemetry (via a small Windows tablet hooked up to the ECU and said tablet held in a tank bag with see-through map window), I can see what the fuel injection system does while I ride. Every time I snap the throttle, the system goes into open loop and does not read the oxygen sensors' feedback. Research says that this is done to prevent engine detonation due to lean-out caused by the lag of the feedback system during sudden engine acceleration - very critical in an air-cooled Harley engine as it already runs hot. (Yes, too lean a fuel mixture causes engine detonation as well, not just spark advance What causes spark knock and how do you get rid of it).
One more point: the system goes into open loop operation at 90 and 100% throttle operation as well. Rationale is the same: the engine accelerates too quickly and the lag in feedback will cause detonation. Not sure if this applies to Honda's system as well but I suspect it does.
In summary, the BoosterPlug makes changes to the open loop operation of the fuel injection system. What this translate to the rider is better throttle response (when you snap the throttle open/close rapidly), and improved low rpm rideability (less "snatchy" throttle).
And there's better fuel mileage too! Although YMMV on the fuel mileage. I most definitely got a slight mileage improvement on my Harley when I enriched the fuel map (similar effect done by the BoosterPlug). My theory is that making it run richer during the snap throttle moments adds to the fuel efficiency. This means a rider with very smooth, controlled throttle roll-on would probably not enjoy the same benefits, however.
I have not installed the BoosterPlug on my daily-ride NC750X, but it's on the upgrade list. Yes the fueling of the NC is already very good, which is why it is a low priority to me and more of a want! The want comes mostly whenever I perform a highway overtake maneuver on gear 6 and the engine hesitates for a bit.