Full-side-facing
Gyaku-zenketsu dachi (literally: "reverse-front stance") are full-side-facing positions are used to literally broadside your opponents, allowing you to wale on them with full hip rotation and stance shifting, without changing your facing direction. This is only useful after establishing dominance, a situation which occurs in our advanced kata and at the end of fights.
To enter a full-side-facing, your feet must be two shoulder-widths apart, and lunging 90° away from your opponent. The bent knee carries 70% of your bodyweight. Your knees and toes point diagonally in the direction of your bent knee. Alternately, enter a front stance, and rotate your torso a 1/8th turn towards your trailing leg. Full-side-facing is similar to terza (third guard) from Italian rapier fencing.
[Photos of full-side-facing from the front and side]
Everything in this world is a trade-off, and stancework is no different. Entering a full-side-facing augments power, but hinders your mobility. Full-side-facings should be used sparingly by advanced students or those with enough experience to make this sort of cost-benefit analysis in the heat of the moment.