The valve routes a small amount of exhaust gas back into the intake manifold diluting the air-fuel mixture that cools the combustion process. The result is less oxides of nitrogen emissions.
The EGR valve gets the vacuum from the carburetor port but that vacuum port alone is insufficient to open the valve. As the exhaust gas pressure increase due to increase in engine rpm, the extra boost is applied to the other side of the diaphragm that lifts the valve open.
I have never worked on the 230 gas engine. I do not know what else I can tell you.