Here's a very rough idea of what I'm talking about.
The top one is the balanced valve. The bottom just the normal poppit valve for reference.
With the balanced valve, air is pushing from both sides of the valve pin, keeping it "balanced". The spring (not shown) ensures it stays closed. With the balanced valve pin, the spring is the only force the hammer works against.
With the standard valve pin, air is just pushing the pin closed.
I can make a real diagram with measurements if you need it.