Camera Parts: Rewind Release
A rewind release button is one of the camera parts found on both manual and automatic film camera. However, it's exact function differs slightly between the two.
Rewind Release on a Manual Camera
When film is loaded in a camera, winding sprockets pull the film from the cassette onto the take-up spool. The gearing in these prevents the film from going the opposite direction. However, once the roll is complete, the gears must be release so the film can be rewound. A rewind release button disengages the gears on the take-up spool and the winding sprockets. Once depressed, the film can be rewound into the cassette using the film rewind knob.
A second purpose for the rewind release button is if a photographer wishes to rewind their film before completing the roll. Like before, this process requires pressing the rewind release button while rewinding with the film rewind knob.
Rewind Release on an Automatic Camera
On automatic cameras, this is usually referred to as a midroll rewind button. It's purpose is the second purpose listed above: rewinding a roll before it is complete. Usually, the button needs to be depressed for some time before rewinding begins. This is to prevent accidental touches.
Automatic camera will rewind a finished roll automatically, so no button press is needed for normal operation.