The Timer explained
In the Timer tab, you can choose between Timer or Stopwatch mode.
The Timer helps you to push yourself when you're fighting procrastination. When enabled, you must commit to working on the task for the preset amount of time.
If you give up before the Timer is finished, you should not claim the credit. Therefore, the session won't be added to your stats or your calendar, and the Break Timer will not start.
However, if you complete the task before the timer is finished, you may mark the task as completed. The session will then still be added to your stats and calendar, (if enabled).
If you don't need the extra push of the Timer, you could use the Stopwatch instead.