Prohibited techniques

Judo prohibits techniques in two quite different ways, and the difference decides what you should do about them. A few are barred outright because the person receiving them cannot fall safely. Others are barred by contest rules while judo carries on teaching, grading and demonstrating them. Both are listed below, kept apart on purpose.

Never to be applied

These are documented so they can be recognised and refereed, deliberately without step-by-step instruction. Attempting one in competition is direct hansoku-make; do not practise them in randori.

Ashi-garami, entangled leg lock, Kansetsu-wazaProhibited in competition and free practice: judo permits joint locks on the elbow only, because the knee gives way without the graduated warning an elbow gives. Ashi-garami is preserved in katame-no-kata as formal study.
Do-jime, trunk squeeze, Shime-wazaProhibited act under competition rules: squeezing the trunk with crossed legs (scissoring the body) is penalised with hansoku-make. Unlike most banned techniques it has no legitimate contest history to preserve; it is listed so it can be recognised and stopped.
Kani-basami, crab scissors, Nage-wazaProhibited in competition since the 1980s after serious knee and leg injuries to receivers; attempting it in shiai is direct hansoku-make. It remains part of judo's historical canon and study.
Kawazu-gake, one-leg entanglement, Nage-wazaProhibited in competition: falling backwards with a leg entwined around the opponent's leg risks severe knee and ankle injury. Named for the sumo wrestler Kawazu, and older than judo itself; it is retained in the vocabulary so it can be recognised and refereed.

Barred in competition

Contest rules bar these, but judo still teaches them and federations still examine them: several are on the British Judo Association's dan grade syllabus. Each keeps a full technique page, step by step. Read the restriction on the page before taking one into a contest, because it is rarely a blanket ban and the wording matters.

Kata-guruma, shoulder wheel, Nage-wazaProhibited in competition in its classical form, where tori's hand passes between uke's legs: the IJF bars any direct attack on the legs with the hands or arms. A version that takes uke across the shoulders without the legs being touched is legal and is thrown in contest today, so the restriction is on the form rather than on the technique. The BJA sets kata-guruma as a 4th Dan fundamental skill.
Sukui-nage, scooping throw, Nage-wazaProhibited in competition in its classical form, which scoops uke's legs from behind: the IJF bars any direct attack on the legs with the hands or arms. The technique is unaffected outside contest, and the BJA sets it as a 5th Dan fundamental skill.
Kibisu-gaeshi, heel reversal, Nage-wazaProhibited in competition: catching the heel is a direct attack on the legs, which the IJF penalises with hansoku-make. The technique is unaffected outside contest, and the BJA sets it as a 3rd Dan fundamental skill.
Kuchiki-taoshi, single-leg drop, Nage-wazaProhibited in competition: taking uke's leg with the hand is a direct attack on the legs, which the IJF penalises with hansoku-make. The technique is unaffected outside contest, and the BJA sets it as a 4th Dan fundamental skill.
Morote-gari, two-hand reap, Nage-wazaProhibited in competition: the IJF bars any direct attack on the legs with the hands or arms, and taking both legs is the plainest case of it. The technique is unaffected outside contest, and the BJA sets it as a 5th Dan fundamental skill.
Waki-gatame, armpit armlock, Kansetsu-wazaProhibited in competition when applied from a standing position: the arm is trapped under tori's armpit and the elbow has nowhere to go, so uke cannot submit in time, and the IJF penalises it with hansoku-make. On the ground it is a legal armlock, and it is taught, demonstrated and examined: the BJA sets it as a 1st Dan fundamental skill.