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Well there are different systems but the basic concept is the same. I'll explain the system I know the best, which is a mid 80's small block chevy.
There is one vacuum source coming from the underside of the carburetor, which sees manifold vacuum. This goes to a solenoid, which can cut off vacuum when cold. The vacuum then goes to the top of the EGR valve. The valve is normally closed, but when a vacuum is pulled on the diaphragm the valve opens. When that happens it opens a flow path for the exhaust gases to enter a second port to the manifold, to be sucked into the cylinders again. The flow is regulated by the size of the opening and flow characteristics, it isn't really modulated.
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