I'd love to know how that one is accomplished. you normally have to use a filter to make it deinterlace back to a full progressive file. it certainly doesn't to my knowledge deinterlace on the fly on its own without being told too.
if it did that, and you wanted to keep a source fil interlaced (for whatever reason) you would have to add a switch or command to your AVS file to NOT deinterlace automatically. no such mention of that on the avisynth.org site.
in addition, if it did do this on the fly, then the likes of Donald Graft wouldn't have bothered to create an avisynth deinterlace plugin surely?