Hey wim,
I'm not sure i know what exactly you are asking? If there is a time point in the video you can point to that can help describe would help.

I'll guess that you are talking about the blend of the long curves to create the new trim curves?

If thats it, then i didnt blend edges. I copied and pasted them to get curves. Then MoI has a mechanism that knows where you "pick" on objects. So by picking the 2 ends near each other, it knows those are the 2 ends i am targeting.

If thats not it, then more info is needed.