There is a lever on the lens which you move will open and closes the aperture ring. The camera body uses this lever to actuate the aperture blades.
If you turn the aperture ring, the aperture opens up. If you now try to move the lever, you will find that the aperture can only move from wide open to this current aperture set by the aperture ring. (No longer can the lever close down to the min aperture.) Thus, this will limit the aperture control of the camera body on the lens. To prevent this from happening, the camera will not work unless the aperture ring is locked at the min aperture (in auto metering modes: A, S or P).