At idle the vacuum is at the highest point. This vacuum pulls up the economizer diaphragm. The economizer diaphragm is used to push down on the economizer block power plastic stem. As the rpm increases, the vacuum decreases and allows the economizer diaphragm to push down on the plastic valve opening the check ball. This allows additional fuel to enter the main well.
The late model Holley 1920's used a 2 stage power enrichment. The power valve lever on this one has two stems, which are used to open two check balls in the economizer body.