When you press the shutter the camera will send a signal to the hotshoe regardless of there being a flash in place (unless your using a custom function that stops the signal being sent like my D2x has).
With manual transmitters fitted to the hotshoe they have a single contact point (the one in the middle of the camera's hotshoe - the other contacts are for TTL information). This single contact is the contact that fires a flash when the shutter is pressed. That's why with many shots featuring OCF there is obvious flash used but the exif doesn't register it because the camera didn't have any TTL contexts linked that would register info into the exif. Think that's how it's best explained.....