Based on what CaptainBigButt has written and what CaptainBigButt, Doktor Haus, and Splinks have produced for their L4D2 addons that use the trick, I wrote a tutorial:

Proportions Trick[sites.google.com]

It includes links to CaptainBigButt's articles on the subject and the L4D2 addons that use the trick at the time the tutorial was written.

:)