Why does the Valsalva Maneuver help alleviate my sinus pressure?

I have had 2 sinus surgeries with no improvement in symptoms.  I do NOT get congestion, but rather chronic inflammation, blurry vision, facial and nose-bridge pressure, and lethargy/brain fog.  

When I hold my mouth shut, pinch my nose, and blow outward (Valsalva Maneuver) the pressure goes away, I can think straight, and my vision improves.

I understand that this is reducing the pressure in my sinus cavities.  The question is: Why?

My life is going downhill fast and I'm trying to figure out why I keep having sinus issues.  ENT doctors are of no help and attribute it to allergies, even though I tested negative to everything!  

What is causing my facial pain (with negative CT) and why does the Valsalva Maneuver work?  I am so depressed with this condition because I can't think straight due to the never-ending pressure.  It is also worse after I eat.  Could this be mucus based?

[lowbrainpower]

I should also mention that I am dizzy ALL THE TIME and my ears + inner ear have been checked out and are fine.  I know the feeling of sinus pressure/pain/infection and am positive this is it.  Would an infection of the ethmoid (behind the eyes) sinus cavities give these symptoms?  I do have some fluids behind my eye, but the ENT says it's nothing to worry about and would not cause these symptoms (facial pain, pain behind the eyes, dizziness, etc.)

Thanks!