I assume my post was deleted due to the link and the awesome competition my website poses.
Anyhow, the drinking straw analogy above is not quite accurate.
This analogy would be useful if we were trying to suck water into the attic through the soffit vents.
But most of us, maybe all, don't live in a house surrounded by water that we want to suck into our houses. We prefer to keep the water out.
So, instead of trying to suck water into the straw, try putting the end of the straw into "air".
Now,place a finger over the bottom of the straw and inhale. Feel the suction?
No, it won't be quite as strong as if there were no hole higher on the straw, but, if that middle hole were restricted in any way, say a cap or screen or even negative pressure from wind blowing over the outside of the straw, the difference in suction would not be significant.
Go ahead, try it for yourself.
Leave your gable vents open.