"I don't understand the metaphor of the why however."
Secondary stroylines are handy when you want to change the duration of the shots above your primary storyline or ive you make modifiations of time to your primary storyline.
They are also nice for lining up titles over multiple shots, etc. or if you want to move sections of Brol around.
Once you start using them they are great. It's like being able to picture lock sections of the edit that you like and want to move around in a group. It becomes one object instead of twelve shots.
Compund clips are cool this way too because you can name them.
Its such a custom way to work.
David
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The shortest answer is doing.
Lord Herbert
http://vimeo.com/battistella