Here is a beautiful example of Kawazu gake (Leg Entangled Throw) which is classified as a side sacrifice throw and categorized as one of the four Kinshi waza (illegal techniques). Keen eyed observers will notice the similarities of Kawazu gake with Shuai Chiao's (Chinese wrestling) "Leg Twist" technique. Many teachers hold this as a "secret" technique in Chinese martial arts for no good reason.
Techniques such as this fascinate me as they span the martial spectrum with no regard to "styles" nor "systems." Riding high purely on principle. Shame on Judo for making it illegal!
I guess this is an illegal throw because to make it effective one must drive the opponents head into the ground. As with many "illegal" techniques I find it silly. We cannot continue to make techniques illegal simply because our opponent refuses to accept a fall! Imagine if we made Uchi-mata illegal because of the propensity of destroyed knees. Is the throw intrinsically dangerous? No. Not at all. But when your opponent/partner resists... well... the knee loses. Cause for a ban? No. Not at all.
Also found in Sumo under the same moniker:
Leg Twist from Shuai Chiao as demonstrated by John Wang's students: