[raffriff42]

I read the whole thread and part of the linked one and I still don't know what TWriteAVI is or what it does.
Never mind, Reel.Deal has summed it up nicely:

http://avisynth.nl/index.php/TWriteAVI

Serve AVI file to program requesting it as well as write an avi file. Useful for speeding up 2 pass encodes at the cost of hard drive space.

Now, how do I use it? You just load the script, and it writes the whole file to disk?
How would it be used in a 2-pass scenario? How would it be used to create a lossless copy?