Vintage Texturing
Hi/lo, erased technique, vintage...these are all names for processes that're becoming more popular in the production of hand-knotted rugs.
What is it? Basically, it departs from the basic flat-top of the rugs pile to create different heights in the pile.
It may be to make the rug look older. It may be incorporated into the design just to give it texture. It may offset elements in the design to give them prominence, make them pop.