When working without turning your work, for instance in joined and unturned rounds or continuous rounds, the back third loop (BTLO) of a half double crochet stitch (and also a yarn over slip stitch – YOSLST) is always located at the back of the stitch. It’s the third loop, or extra loop, sitting just behind the two usual front and back loops.
The following tutorial photos show the HDC stitch, but the same principle applies to the YOSLST as well!