Knee joint kick with the knife-edge of foot. Called both kansetsu geri and sokotu geri.
Foot position: Big toe pulled back. Rest of toes pulled down. Flex foot back toward knee and turn on side. Kick is with edge of foot. Contact is with edge from heel to the bony protrusion on the side of foot.
DO NOT kick with front part of foot side lest you break it on knee.
In application this is a snapping kick, usually practiced at 45 degrees to front. Hip rolls slightly.
Alternative action is to kick like a side kick, thrusting through the knee with the heel/bottom of foot, with full hip rotation in the process. Think of it in full contact as a stamping kick with the heel kicking down and through (about 4-6″).
The point of contact for the knee joint kick is about 3″ above either the inside or outside of the knee.
Please note that the foot position in the upper picture is NOT CORRECT for goju practice. Big toe up, rest of toes down, not like shown with all toes pulled back/up.