I think the shirt he's wearing in the video is a folded in half shirt, very controlled, manipulated mirror image scrunch.
I'm sure there's a "trick" to doing a really good mirror image, but I haven't been able to consistently do them. I often get mirror images when I'm not trying for them.
I tend to dye the cloth damp and put the soda ash in the dye but I think producing a good mirror image might be to soda soak the tied item, dry it (or almost dry), and then dye it with slightly thickened dye.
Here is a tie artist who does great mirror scrunches:
http://www.etsy.com/shop/bigbluedyesk. taltre