The Elastomer Valve (or as we call it Umbrella Check is used to control gas intake to the diaphragm and the discharge.
When the throttle is let up, the diaphragm creates a sort of suction which pulls open the Elastomer valve and allows the diaphragm reservoir to fill up.
When you press on the throttle the diaphragm is pushed towards the carburetor which pressures the Elastomer valve closed. The gas is then forced through the discharge hole in the carburetor body housing and up through the venturi and main discharge.
As you let up on the throttle, the return spring presses the diaphragm forward and again opens the Elastomer valve.