Cameras with off-the-film metering measure the film reflectance at every shot. The first metering is done before the shutter opens, either with a speckled curtain (OM2 for instance) or via an auxiliary mirror behind the main mirror (Nikon F3). When the shutter opens a second reading is made from the exposed film and the ratio of the shutter closed/reference reading and shutter open/live reading gives the film's reflection density. This density, in combination with the camera's ASA setting, is then used in setting the trigger for integrating light for either TTL flash or automatic time exposures.
Hi, Nicholas! Darkroom Innovation...I remember well from my days with a Beseler VXL!