My understanding of it is that it increases top end power, which is the part of the power band where a kart normally runs. The downside is that it lowers midrange power; midrange is beneficial to the motocross guys, but not so much to us. So, we remove the valves and plug those ports to change the power band to what benefits us on track.
I am sure someone could more accurately explain it, but I think that is the overall idea…
2010 Spirit [Birel] SP32 // SRS CR125 - Stock Honda [#08]
Hill Country Kart Club - New Braunfels, Texas - http://daledaugherty.wordpress.com/