To make a half double crochet stitch: yarn over once before inserting hook. Insert hook, yarn over and draw one loop through. (You will have three loops on your hook.) Yarn over again and pull through all three loops.
Here we have a swatch of half double crochet.
The red is a swatch of single crochet. Notice the size difference.
To learn how this stitch is made, watch the video on the directory of stitches page.