I'm going to give you instructions assuming that you're right handed. If you're left handed, you'll need to do exactly the opposite. When doing a regular half double crochet, you: YO, insert hook into next stitch to the left, YO, draw a loop through, YO and draw through 3 loops.
When working a reverse half double crochet, you: YO, insert hook into the next stitch to the RIGHT, YO, draw a loop through, YO and draw through 3 loops.
In reverse crocheting, you will work in the opposite direction than you are used to. This takes some practice and may require that you hold the hook differently. Reverse crochet works wonderfully with single crochet, half double crochet and double crochet. It is usually used as an edging since you are unable to crochet on top of it.