I recently learned this technique from Becky of Grammy Dirlam and was happy to make a video tutorial!
When you work in the 3rd loop of a half double crochet normally you are left with a large gap in the fabric. This technique involves you not only working the 3rd loop but the forth loop as well. The creates a more sturdy fabric with no gaps at all.
Spike Half Double Crochet (Shdc): Yarn over, insert your hook in the stitch one row below, pull up a loop, yarn over, pull through all 3 loops on the hook.
Reverse Single Crochet (rev sc): Insert hook into the previous st (to the right), yo, pull up a loop twisting the hook around to face upwards, yo, pull through remaining 2 loops on hook.
*If you are left-handed you will work the rev sc to the left.