Learn o-uchi-gari, the major inner reap in judo: backward kuzushi, the reaping motion and milestones, honest and safe with breakfalls and supervision.
The honest answer: o-uchi-gari is a throw that feels easy and therefore has to be practised especially cleanly, because your partner falls straight backwards. You reap your partner's leg from the inside to back-outside while you lead him with your upper body in the same direction. The German Judo Federation names o-uchi-gari, next to o-soto-gari and de-ashi-harai, as a suitable foot technique already for 7th kyu [1].
The nice part: it needs little strength, but timing and body control. The delicate part: the backward fall is exactly the one where breakfall skill decides everything. Expect several weeks until you bring the reap and the upper body together. That is a practical rule of thumb, not a study.
One hand holds Uke's sleeve, the other his lapel. You stand upright, knees slightly bent, weight on the standing leg. Milestone: you can lead Uke backward without yanking, without tipping backward yourself.
With pull and push you bring Uke so far back that his weight is on his heels. Without this tipping point the reap is ineffective. Milestone: Uke stands on his heels and feels that he can barely keep his balance.
Practise without a throw: with the inner foot you reap an imaginary leg from the inside to back-outside, standing leg bent, upper body upright. Milestone: 10 clean repetitions per side without wobbling.
You practise entry and reap on the partner without completing the throw. In the same time judoka managed more uchi-komi repetitions in o-uchi-gari than in seoi-nage or harai-goshi, and a repetition costs less energy than in seoi-nage because no trunk rotation and no deep knee bend are needed [3]. That makes it a good practice throw. Milestone: 20 repetitions in a row in which Uke really goes off balance every time.
With your coach's approval you throw for the first time. Important: the pulling hand stays on the sleeve and accompanies Uke until he lies safely. In competition an ippon requires speed, force, landing on the back and controlled landing [2]; as a beginner control comes first. Milestone: Uke lands with chin on chest, hands slapping, without impact on the back of the head.
O-uchi-gari is a throw with a backward fall, and exactly there the back of the head is at risk. In a dummy trial with one experienced thrower, the rotational head acceleration in o-uchi-gari was around 1,700 to 2,100 rad/s², but the linear acceleration was 120 to 149 g, clearly higher than in o-soto-gari (64 to 87 g) [4]. That is a hint that even this "easy" throw must be taken seriously, not a reference value for real partners. Novices showed different movement patterns in the backward fall after o-uchi-gari than after o-soto-gari, a hint that each technique needs its own breakfall training [5]. That breakfall skill protects was shown in a study with two judo experts: with correct ukemi the head accelerations were significantly below those of a dummy without ukemi [6]. In national-team candidates the force on the head after o-uchi-gari tended to be lower than after o-soto-gari (not significant, but with a large effect size), and the more body surface and arms touch the mat early, the more gently the head is decelerated [7]. You only throw partners who master ushiro-ukemi securely, only on tatami and only under supervision. This guide explains and motivates, it does not replace supervised training.
Let your partner feel the pulling hand: after the reap it leads him to the landing. For Uke it feels like a handrail and lowers the fear of the backward fall. A calm pulling hand is half of the safety equipment.
And practise both sides. From 7th kyu on, the German Judo Federation expects throwing to both sides [1]. If you can only do o-uchi-gari on your favourite side, you are easy to read in randori.
Technically yes: the German Judo Federation names it as an example for the first throws [1]. In terms of safety: only with secured backward breakfalls.
In o-uchi-gari you reap the leg from the inside, in o-soto-gari from the outside. In a study with novices the falling movement differed clearly after the two throws; a greater extension momentum and a lower head position were measured for o-soto-gari [5].
Yes. The rules allow o-uchi-gari and score it even if the leg is entwined with the partner's leg during the throw. Kawazu-gake, on the other hand, is forbidden [2].
Start without a throw and without speed. Only when Uke goes slightly off balance in every repetition do you increase speed and number. How many repetitions your coach asks for is individual.
Ko-uchi-gari: The Minor Inner Reap for the Moment the Heel Carries the Weight