Purling can be used combined with plain stitches to create the "stockinette" stitch which is shown at the end of the video.
The plain stitch is stitched at the back of your work, and the yarn is held at the back too. This should be done on the first row.
On the next row you purl: the purl stitch is stitched at the front of your work, and the yarn is also held at the front of your work as this video shows. After the first two rows you continue in this way: knit, purl, knit, purl; ect, to create the stockinette stitch.
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There is no audio btw. (less info)