Part of it will always be a too lean mixture. Fitting a BoosterPlug to increase start and Idle mixtures with 6% will make the bike start easier and idle stronger.
Does the BoosterPlug not just operate under acceleration?
Nope, right from the start.
Are you sure? My understanding is that, on a closed loop set-up, it will only interfere during throttle movement.
"In conditions where you maintain constant RPM and throttle opening, the feedback from the lambda sensor will adjust the mixture back to the original level, and our small tuning device will sit idle and wait for something to happen. This is fine – you don't need the richer mixture at level speed.
As soon as RPM or throttle opening moves, the fuel map will shift horizontally or vertically on the 15 x 15 grid, and the lambda sensor feedback will be temporarily disabled. This means that the ECU runs open loop for a short period every time we change RPM or throttle, and the enrichment from the AIT sensor modification kicks in exactly at this point."
I may have it wrong, so hoping Jacobsroodt will educate. 
It took me a while to figure this out as well. This is the correct version:
"As soon as RPM or throttle opening moves, the fuel map will shift horizontally or vertically on the 15 x 15 grid, and the lambda sensor feedback will be temporarily disabled. This means that the ECU runs open loop for a short period every time we change RPM or throttle, and the enrichment from the AIT sensor modification kicks in exactly at this point."
As long as one block on the grid is used on an continuous basis, the lambda sensor has time to provide feedback and correct the mixture. But as soon as the fuel map shifts from one block to the next, the lambda sensor is too slow to make adjustments and takes it out of the equation. The job of adjusting the mixture correctly is then left to the 3 remaining sensors: oil temp, air pressure and intake air temperature (IAT) sensor.
All these sensors makes adjustments to the air/fuel mixture. E.g.: at 0°C, the oil temp sensor will enrich the mixture by 35%!
The AIT sensor pushes the mixture back to a very lean 14.7:1. The BoosterPlug adjusts the IAT sensor values to result in a 6% richer mixture - whenever there is movement between the blocks.
The higher the rpm, the bigger the blocks and the longer time is spent in one block. But the lower the rpm, the smaller the blocks, and more movement takes place on the grid. At idle the values jump around rapidly between blocks all the time. BoosterPlug makes Idle 6% richer. This is why a bike will start easier, "walk" stronger at idle and accelerate better from low rpm.
The BoosterPlug is most effective in the bottom half of the rpm curve where the blocks on the grid are smaller and more movement takes place - up or down the grid. That is why the BoosterPlug works so well. The bike now runs at 13.6:1, which is the correct stoic mixture, and not at a lean 14.7:1. Throttle response is better, snatchiness is removed, the engine runs cooler (fuel cools down the cylinder) and surging is cured. Because the mixture is richer, the engine decelerates and settles back into idle smoother - resisting stalling and giving the bike a more forgiving character.
The net effect is a wider usable rpm range - the same work can be done at lower rpm.
Sooo, the boosterplug works under acceleration, deceleration, at idle and on starting

.