The latest video from Rainer König discusses countdown and relative timers. These aren't the clock-in, clock-out timers—used to log time spent time spent on tasks—that you may be used to. They're a separate facility that I didn't know about before I watched the video.
You start the first type of timer, the countdown timer, by typing 【Ctrl+c Ctrl+x ;】 after which Org asks you to specify a time and then starts counting down from that time. When the time expires, Org mode gives you an expiration notice. In the video, Org pops up a message box. In my Emacs configuration, I get the message in the minibuffer. In either case, the current time remaining is shown on the mode line.
As König explains, this is useful if you want to work on a task for a specific amount of time before moving on to something else. You can pause and restart the timer with 【Ctrl+c Ctrl+x ,】 and stop it entirely with 【Ctrl+u Ctrl+c Ctrl+x ,】.
The second type of timer, the relative timer, is useful for taking notes with times as some event is taking place. For example, if you are taking notes on König's video, you can start a relative timer and have your notes keyed to the video by the elapsed time. It's the sort of thing that Sacha Chua does with the notes for her videos, although I don't know if she uses relative timers to do it.
To start a relative timer, you type 【Ctrl+c Ctrl+x 0】 and the timer starts from 0. You can input a starting time by preceding the sequence with the universal argument.
When the timer is running, the current time is shown in the mode line and you can insert the time as a string with 【Ctrl+c Ctrl+x .】. More usefully, you can make an entry in an item list with the current time by typing 【Ctrl+c Ctrl+x -】. You can add subsequent entries in the list by typing 【Meta+Return】 as usual. That's perfect for the type of transcripts that we discussed above.
You can use the same sequences to pause/restart/stop the timer as you did with the countdown timer. If you're using the excellent which-key, it will prompt you for these sequences if you can remember the 【Ctrl+c Ctrl+x】 prefix. That's not too hard because many Org actions of this sort (including the clock-in/out commands) start with the same prefix.
This is a really excellent video and you should definitely watch it. It's only seven and a half minutes so it's easy to find time for it.