Your engine is quite simply a pump. It sucks air in one end and blows it out the other. However, along the way is grabs some petrol, which atomises into the airflow. this is then squeezed into a small chamber and ignited. The ignited mixture heats up and pushes the piston down, and the hot gasses from the explosion then exit through the exhaust pipe.

Now, to get max power there's way to many parameters to go over in one post, so let's just stick with the air and fuel.

I was told many moons ago by a very creditable engine builder that the trick is not to try and lean the mixture off to get power, but to try and get as much air through the motor as possible and then match the petrol with the air in a perfect ratio.

A good example would be that you've got the bike on the dyno and it's making "this much" power with the air filters in place. The problem is that it's a tad rich. So we can either lean the fuel off to get the right air/fuel ratio with the filters fitted, or we can try to get more air in there by taking off the filters. More fuel and more air (more oxygen) means a bigger bang so more power.

That is assuming that the filters are the restriction to the airflow of course, and not something else like tiny reed valves etc etc.

When testing pod filters on the brake I could always find another 2~4 ponies by throwing the filters in the bin and running open carbs.

The NSR airbox isn't all bad, but it does need some major modifications to get the best from it.