Like a cold knife through hot butter... (Nine Ball M92F Gas Router)
Air. When it's flowing it moves almost the same way as water does and anything in it's path will disrupt the flow and usually cause resistance. There's a whole load of scientific mumbo jumbo about this general topic, known as Fluid Dynamics (and it's sub-discipline for air, Aerodynamics), and whilst we mostly don't need to care about the majority of it, it's important we understand the super basics for this article (actual scientists please spare us for the inaccuracies).
In the image above, we see a fine example of Aerodynamics in effect. The red parts showing where airflow meets heavy resistance, with the blue parts showing where there is less. Generally, the smoother the surface and rounder the edges, the better air will travel over the object and the less drag it will leave behind it (you can find out why here). Much like a cow, and our precious BB's with their microscopic bumps and craters that all do a great job to disrupt the air traveling over them, there are other parts of our pistol that are greatly dependent on how airflow interacts with them.
The part we are most interested in lies concealed within our magazine and plays the critical role of delivering our propellant; the magazine gas router (part SC-58).