This is a provisional cast-on, also known as invisible cast-on: one that can be easily undone to expose live stitches, and then knit from seamlessly. Usually done with a second piece of yarn that is ultimately removed. It can also be done with the flexible cable of a cable needle, allowing the provisional stitches to be knit directly from the cable needle.
Note: When you go to knit off of this bottom edge, every other stitch (ie. on the cable needle) will be twisted. You must knit (or purl) through the back loop of the twisted stitches.
Provisional cast-on is basically a temporary cast-on which you can later remove to reveal live stitches to knit from. There are many applications. Say you are knitting a pair of socks and only have a certain amount of yarn and you're not sure how high to make the first sock so that you don't run out of yarn before completing the second. You could use the provisional cast-on to begin the socks, then when both socks are knit, expose the live stitches and knit each to the same height using your remaining yarn. This ensures both sock are the same height and you don't run out of yarn. Beats running out of yarn before you finish the second sock.
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