The gas cap has several functions, I won't mention all, but most. The poog problem is due to the alluminum corrosion from gasoline and water in the air. The way the cap works, It is suppose to keep the fuel in and also allow the tank to breath in and out due to temps atmospheric pressures etc. The valve in there is the part that tries to keep the fuel in even if tank was upside down due to a accident. The smallest parts of the vent line from tank to vent tube under bike is in the cap at just under 1/16 inch (valve is even smaller with several holes) If the corrosion gets in the valve it will render it useless. You have to remove the valve completely to make a good effect to the poogs. Like I said before the line is small and can still be blocked sometime but not nearly as possible. I opened up those lines more on mine to prevent any issues later and even included an filter to prevent some contamints.
The valve opens and closes on whichever side the air needs to breath. You can think of the valve as a sypon or check valve if it doesn't work at times. Air is compressable, liquid is not. If you open the line more it will breath more and the fuel in a sense will fall into the tank in such a splash inside. The fuel will not beat gravity, the cap is higher. You could get a couple drops but not much motorcrossing(Ever notice the much higher vent tube coming out of their caps?), Normal road riding, about nil.