BLOsc (Back Loop Only Single Crochet)
A standard single crochet worked exclusively under the rear horizontal loop at the top of the stitch, leaving the front loop unworked on the front surface of the fabric.
Insert your crochet hook under only the rear strand of the V at the top of the stitch. Yarn over, draw up a loop, yarn over, and pull through both loops on your hook.
The unworked front loop left by BLOsc becomes the anchor point where dropped Front Loop Only double crochets (FLOdc) attach two rows later.