Interesting to see the movements. Using as a timer for any length of time would screw up the time keeping due to stopping the whole watch running. I presume you would just time short increments and it wouldn't matter too much.
The Helvetia 'Stop' 822 movement uses an almost as simple but more elegant process. By pressing the button top left the two train wheels at the top are moved upwards and disengage from the centre seconds wheel to stop the seconds hand.
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