うちまたは じゅうどうの なげのわざ(throwing technique) です。うちは 英語で home, inside です。
または 英語で thigh です。
うちまたは はやります。でも やさしくないです。
It belongs to ashi-waza techniques (foot techniques), which belongs to nage-waza.
In this technique, the opponent falls foreward.
There is two ways to perform this technique : one using your hip to sway your opponent's inner thigh (the left one if you are using a right-handed guard), one using your leg to sway your opponent's inner thigh (this time the right one). If the last way is poorly executed, it can quite be hurtful to your opponent's manhood.
The first one is not the original way, it's harder to perform but truly beautiful.
The second way is easier if you are taller than your opponent, it's generally more efficient. You often see it in competitons.
The reason it's so popular is that it's quick, easy to put in a combination, and if it fails you can use another ashi-waza technique that makes the opponent fall backwards (ko-uchi-gari for example, cf one of the next posts...).
I wanted to put more japanese in this post but I'm still not good enough, sorry :)
Check this link out to see David Douillet using uchi-mata, it was one of his favorite tehcniques, the referee gives him yuko but the japanese commentators don't seem to agree with it...
Vocabulary :
はやる : to be popular (not sure that I used it correctly though)
