Browsing on Youtube I found this fascinating method:
The basics are:
- duplicate your video clip
- invert its colours
- set opacity to 50%
- slide timeline by a little (1 frame for rapid movement extration, 1 second for less rapid)
I’ll give it a go when I’ve got some stable enough video.
EDIT:
I gave it a quick go. I’m not wholly impressed and will have to investigate more.