[Therion] Additional morphing points (type "extra") explained

[Benedikt Hallinger]

Hello,
i struggle to understand how the extra morphing points are meant to be used.

How do they work technically? How do i correctly set them?
Are they linked to the nearest station and could i use more than two extra 
points?
Are they linked to LRUD data?

I already investigated the "morohing" example from the documentation (both 
in code and in thbook), so i think it would be good to stick to this example.

Thank you very much!