Machine Weft extensions are a collection of single hair strands sewn together by a machine.  Loose strands from bulk hair are fed through a triple-head sewing machine, adding a reinforced stitch near the top of the single strands.  The stitching of the strands collectively becomes a long weft and two of these wefts are stitched together; this creates a bundle.  Machine wefts are more durable due to the doubling of the weft and can be cut.  Hair is folded for this process and does have return hair (folded over) at the seam.