It operates similar to an exhaust fan. A vacuum is applied to the unsaturated zone to reduce soil atmospheric pressure below atmospheric pressure. When this is achieved, air will flow from the basement into the soils surrounding the basement. Air flow is reversed from that depicted on the previous webpage illustrating venting of the soil atmosphere into a structure. It is this reversal of air flow that will prevent vapor clouds from entering a basement or crawl space.
Air flow is now from the basement into the soil atmosphere.