Could someone teach me how to cut out a portion of recorded motion-detection videos and download it to my iPhone ?

While replaying recored motion-detection video, I tapped on the video camera icon to start copying a portion of video to app's Memory, and tap it again to stop copying at the end of the portion I wanted.

Then, I opened Memory tab and checked the clip copied there, but the time frame of the copied clip is quite different from what I wanted. The clip starts several seconds later than I intended, and so it does not include the scene I wanted to download to my iPhone. I tried this several times, but I got the same results.

Am I doing something wrong ?

If someone knows the right procedure, please teach me.