I know this is an older post, but thank you! You saved the day again 
Why is it that we need to tie the transformed vector to the ribbon facing to get the ribbon to show up in local space?
Not entirely sure what you mean.
We’re essentially interested in the swords “forward direction”, the bone we’re attaching it to already has that direction, but because this is a trail, we want the particles to be left behind.
So the system is world space, but our bones direction is still in local space, so we need to transform it into world space for it to work with the system.
Does that answer your question?