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Note, to disable automatic work entries, it is necessary to leave empty value. If you need a separate option to enable/disable automatic tracker, so that you can use In Progress Issues without automatic tracker, please, let support know at Support Portal.
Mechanics behind the scenes, is as follows. The timer counts real time, while issue is open in browser, and stores temporary value every 10 seconds in browser local storage. All timers can be seen on report page in Time Entry view also.