Runners are meant to do one thing: boost incoming air charge. What happens is the shockwave of the intake valve closing travels back up the intake and ends up bouncing off of the metal up in the manifold. This, if harnessed properly, can actually give you a "boost" effect (similar to an itty bitty turbo). The runners are calibrated to a certain length, so that the shockwave will be coming down, right when the intake valve of another cylinder opens. (Actually this is sort of simplified, the runner is a factor of the runner length needed to produce the boost effect at a certain RPM, since a runner tuned for say, 1800 RPM would need to ideally be 10ft or so long, but if you make it a 2-based factor (x/2, x/4, x/8, etc) of that length, it still works because the shockwave will bounce around.) By introducing variable runner lengths, you can produce that effect at two RPM values, that's what this part does.