I have used the Kinovea countdown timer for tennis strokes, where "0" time was ball impact. I don't believe that I have ever seen this in tennis videos except for my few videos on Talk Tennis.
One issue for me was that I could not remember how to set up the countdown timer or easily find instructions again.
I can view the video and observe when sub-motions begin and end relative to impact and write down the times. That is great. Also, every sub-motion in a serve video can be timed from a single video. Great again.
https://www.youtube.com/watch?v=N4BSPXl5qA4
Go full screen. Single frame on Youtube, use period & comma keys.
Anyway for multiple timelines is there a way when viewing to record
Sub-motion A__time started__time stopped._______(relative to 0 time of a countdown timer.
Sub-motion B__ etc.
Also, is there some way to make a Quick Kinovea app that would demonstrate the countdown timer OR to provide instructions on setting it up_ on the screen whenever "countdown timer" is pressed - anything to see count time timers on all videos for sports analysis.