What is this vac thing for

vacuum, throttle, open, applied, touch, http://img.photobucket.com/albums/v313/cyfi/cfaff9d6.jpg, sucks, slightly, understnad, diaphragm, thing, located, swivel, pushes, body, plate

I dont understnad what this vacuum diaphragm is for. Its located on the throttle body and pushes against the throttle swivel to keep the plate slightly open when there is no vacuum, but as soon as vacuum is applied, it sucks in and doesnt touch the throttle. What is this for?! http://img.photobucket.com/albums/v313/cyfi/cfaff9d6.jpg