When the sun is shining, indicating warmer dryer air, the Intelligent Control Centre engages the fans to draw in this air. The high torque fans are drawing the air from under the house and blowing the mixture of this fresh and stale air mixture out the vents. This causes a negative pressure or vaccum state under the house. The air naturally wants equal pressure, so it draws in fresh air from outside the house. This causes cross flow and turbulant air extracting a mixture of stale and fresh air into the area outside the home. The diagram below shows one simple way to setup the fans, another common way Nator Manufacturing designs the system is using one of the fans as a central ducted circulation fan, this is common in areas where the floorplan in the crawlspace has dead zones where pockets of air are unlikely to even experience flow.