A thumbnail (and thus imprecise) description of the M4-2 winder operation.
It was designed to require the minimum amount of modification to the cameras (especially electrical - Leitz was a mechanical and optical company that always had problems getting electrical stuff right, in that era), and thus depended on some weird mechanical feedback rather than electric signals to function.
You push the shutter button, releasing the shutter, and simultaneously pressing down a long shaft of sheet metal inside the camera, to close the electrical contacts in the winder and send power to the winder's electric motor. The motor then starts to turn and advance the film and cock the shutter. As it turns, it also returns the long shaft of sheet metal upwards against the shutter button to "unlock" the winding mechanism (try manually winding a Leica film body with the button depressed, and you'll see a depressed button locks out film winding, or results in nasty "grinding"). Coincidentally knocking your finger up and out of the way.
In this regard it is rather like the add-on "bump-stocks" for AR15-type rifles, made famous after the mass shooting in Las Vegas, that use recoil to push the button/trigger against your finger.
The motor does not stop when the shutter is fully cocked. It keeps right on trying to move a now-immovable wind mechanism, and having nowhere else to go, the motor itself torques or twists or writhes in its mount (intentionally slightly loose), in the process separating the power contacts, and turning itself off.
Think of a bump-stock combined with an unbalanced washing machine, and you get the idea. SnaprrrrWHOMP!
This is why the M4-2 et seq cameras were given hardened-steel wind-mechanism gears instead of the smoother-operating softer brass gears of the M3/2/4/5. The motor's "thrash" at the end, to break the electrical connection, would have broken or distorted brass gears rather quickly.
It is in fact recommended to NOT use the M4-2 Winder "dry" with no film loaded. The film drag helps "tame" the motor force - but only slightly.
Edited by adan, 11 August 2018 - 05:16.