Alright. Basically, Avisynth is a plugin that allows you to alter your video in different ways using coding. For instance, Crop(0,0,0,0) will crop portions of the clip off, depending on the numbers you put in instead of zeros. Using this program, you can also smooth your footage, sharpen it, stretch it, shrink it, adjust the brightness and color value, increase/decrease the frame rate, etc etc. You get the idea.
Here's the process I was talking about:
1) Load your Avisynth (.avs) file into a program called VirtualDub
2) Make an AVI called the same name as your .avs file. This will be a low-quality file and will be what you'll edit with. I'll explain in a bit.
-Jump ahead in time- 3) After you're done making your video in your editing program, close the program. Take any and all low-quality AVI file you made to edit with and place them in a new folder.
4) Fire up your program again and when it asks you for the files you just moved, refer it to your AVS files.
NOTE: I've only done this trick with Premiere. I'm unsure how this would work with other editing programs like Vegas or FCP. There might be plugins for programs besides Premiere so that they accept .avs files but I can't say for sure.
This is my first time explaining the process to anyone, so I'm sorry if I missed a step or confused you at all.
This guide explains things better and expands a ton more as well:
http://www.animemusicvideos.org/guides/avtech/Hope any of this helps.