This is a fairly ingenious cast-on method. You use the crochet cast on, one of my favorites, but with waste yarn. When you pull out the crochet chain, you are left with your live stitches on waste yarn, easy to pick up and use. Unless of course you use black waste yarn on black live stitches. I managed to pick up all the correct loops; you can kind of see which are the live stitches and which are the waste yarn in the picture above. Here's a Paint-ed version, so you can more easily tell how it all fits together:
The red lines are the waste yarn, the blue loops are the live stitches to be knit together for the hem. You may notice that there is one live stitch for every two rows of ktbl in the hem - this pattern has you cast on half of the required stitches, then k1, yo for the first row. I like this method, it makes it nicer to either knit together or seam the hem later, with one live stitch for every two actual stitches. I also do my hems with ktbl. I'm not sure if it actually makes it fit better, but I learned that from knitting my Rogue and I thought it turned out well, so I'm using that technique again.
You'll probably see this sweater soon. I'm quite a bit further than these pictures would have you think, and I'm being nearly project-monogamous until it's finished. I've only got one simple stockinette sock in my bag as a backup for lectures and readings where the colorwork1 is too attention-consuming but I can/should2 still knit. Hopefully, a FO post with this gorgeous sweater soon!
1 This sweater is Norwegian-esque and almost every row is stranded. You can see the other color, the pink, peeking out from the prior row in these pictures. I cannot wait to wear this sweater, hopefully this winter!↩
2 I have ADD and knitting is a coping mechanism. It helps me focus. I know I've mentioned the fact that knitting helps me focus before, but I'm not sure if I've "come out" on this blog as officially ADD before. I am, I was diagnosed not too long after we came to Indiana, and it's good to know that many of my issues from anxiety to Imposter Syndrome to general disorganization stem from something that is not a moral failing. I may blog about that in the future, but I keep this blog mostly craft-related, so I may not.↩