A crocheter wrote about what a half-double crochet is between a double crochet. Where are the stitches? That's the point of this stitch: there are no stitches to insert the hook into. She was referring to the Crochet Pants pattern.

The stitches are between the posts of the half-double crochet. They sink down between the stitches rather than on top of the stitches.
Why would they do that? It causes the fabric to be more dense and more complex to stretch apart, especially, when draping down a leg.

Tutorial
Summary

Article Name
What is a Half Double Crochet Between The Half Double Crochet
DescriptionLearn what Yarnspirations means to place a half double crochet between the double crochet.
Author Michael Sellick
Publisher Name
The Crochet Crowd
Publisher Logo

Kathy Lapinsky says
Thank you. I always like to learn new (to me) crochet stitches. Your tutorials are very easy to follow.