Fabric diagonally folded (practice with a piece of paper first) stitched in a few places on the long straight edge as shown in inset picture. Beautiful chevron design is revealed after indigo dyeing. Sample made by Townhill Studio.
This is called Sekka shibori which is one of Itajime Shibori dye techniques. It's created by pleating, folding, clamping and dyeing the fabric. The fabric has to be folded and pressed neatly to create nice and precise patterns.