Hittsui-geri (literally: “knee-hammer kick”) is used to deliver devastating blows to the groin and abdomen from a close ranges, to crush the head or solar plexus of doubled-over or kneeling opponents.
Knee strikes are simple. From close range, pull your knee up, as though you were entering a walking crane stance in order to throw some other kick. Point your toes down -- not forward -- when kneeing an opponent. This seems trivial, but this forces your knee to travel forward, and not up, driving into the opponent with an additional 3-4” (~7-10 cm) of extension.
Wen throwing knee strikes, be sure to strike with the top of the knee. Striking with the front or point of the knee can dislocate your kneecap (patella).
[knee strike vids fasts, and slow, from the front and the side]