I'm curious how a slip "allows" more air into this equation? A filter...sure, less restriction allowing more airflow, but an exhaust is a one way street. A slip allows more air OUT, but not into the fuel:air ratio. Lessens the back pressure allowing the exhaust to flow more freely when escaping the cylinder (i.e. the engine doesn't work so hard to push it out). If you want to maximize your fuel/air burn, I'd suggest looking at your ignition, timing, etc. Make sure your fire is hot enough if you're bumping up your fuel to air mixture.
Not hatin, just curious.
That's a great question... Modern sport bikes have all kinds of "tricks" that engine tuners have learned over the years.
When taught about combustion, we're told that intake valves open to let air and/or fuel in during the intake stroke, they close, gasses are compressed during the compression stroke, combustion occurs during the power stroke, the exhaust valves open, gasses are pushed out in the exhaust stroke, and the process repeats. While that's a greatly simplified explanation that works well to explain how things work the devil is in the details.
One trick includes the exhaust valves opening slightly before combustion and the power stroke is complete. There is minimal power loss, and the expanding gasses give the exhaust stroke a head start. The exhaust is literally being pulled from the cylinder rather that having to be pushed.
Another trick, and this is how a slip-on affects getting more INTO the engine, is opening the intake valves before the exhaust valves shut. Exhaust can be treated like any other fluid. As it's is being pushed/pulled out of the cylinder, if the intake valves open prior to the intake stroke, fresh air can be pulled into the engine for free because the exhaust is flowing out the other direction. Kind of like a siphon. If we get the exhaust gasses moving out faster/easier this trick can allow more fresh air and fuel into the chamber sooner. When the intake stroke starts, there's already fresh air in the chamber.
There are hundreds of "tricks" that are being used in modern engines. Each individually affects performance only slightly. But having a fraction of bhp here and a fraction there adds up in the end. Whether or not you can put these extra ponies to work is all up to the rider.