It begins by using the Crochet-On Provisional Cast-On to provisionally CO stitches for the heel (I used a tightly spun cotton yarn), which are inserted at the position of the double increase marker. Whenever I work a crochet chain (which is basically how the Crochet-On Provisional Cast-On is worked, only it’s worked around the working needle), I tie an overhand knot after the last stitch so I’ll be able to identify which end to unravel later.
Inserting the new stitches is a bit of a tricky maneuver. You’ll want to use a circular needle with a very flexible cable in order to knit outward from both sides of the provisional cast-on. This involves making a hairpin turn in the knitting by pulling out a loop of cable as for working the Magic-Loop method.
The knitting feels tight at first but loosens up with each successive row as double increases are worked at the center of the heel stitches.
The provisional cast-on will appear as purl bumps on one side of the knitting (the right side in my example) and chain stitches on the other side of the knitting (the wrong side in my example).