Vacuum is applied to upper EGR diaphragm via a hose connected to intake manifold vacuum. Manifold vacuum is also applied to lower EGR diaphragm (through intake port at base of EGR valve).
When manifold vacuum in lower chamber is insufficient to overcome spring tension on lower diaphragm, bleed valve closes, allowing vacuum in upper chamber to open EGR valve. With engine at idle or under light load, high manifold vacuum applied to lower chamber opens air bleed valve in lower diaphragm. This bleeds off vacuum in upper chamber, keeping the EGR valve closed.