Half Fisherman's Rib - Left Leaning Double Decrease / LLDD 

This video shows you how to work a double decrease when working in the Half Fisherman's Rib stitch.
This is worked on the front side of your work and begins in a K1B column and is worked over 3 stitches.
I like to break it down in steps as follows:
Step 1 - Slip 1 from below Knitwise from left needle to right needle.
Step 2 - K2Tog. In order to work this step easily I like to prepare the stitches rather than try to insert my needle into the k1B and the Purl stitch. Put that is doable. To prepare the stitches slip the next stitch purlwise from left needle to right needle, then slip the 3rd sts purlwise 1 stitch below from left needle to right needle, once completed, now you K2tog.
Step 3 - Pass 1st slipped stitch over the last stitch you just knit.
If undoing these decreases, you must work in reverse order, starting with Step 3.