Back in the early 1970's, before Pre-Faded and Stone-Washed jeans were in circulation, the denim bought then was as stiff as cardboard when new and the shade of denim at it darkest. It would take months of wear and washing to get them worn-in properly.
To speed the process up, a couple of homemade techniques were used.
One was bleaching, although that sometimes just made the denim blotchy.
The other was to use sandpaper.
Smoothing the jeans with sandpaper greatly increased the fade as well as making the denim more comfortable. This then lead to the "Fade" itself becoming the fashion, not the pricey jeans, and the way Dundonians showed off their fade was to rip off the brand labels, revealing the original untouched dark blue denim underneath, thus highlighting the extent of the fade.
In some extreme cases, they totally removed the back pockets!!
The fad was pretty short-lived and eventually FADED out when manufacturers got wise and you could buy such denim ready to wear in the mid 70's.