1. Kiraku-ryu jujutsu has, as one of it's proudest stories, a match between a top-ranked Kodokan judo man and a Kiraku-ryu practitioner (in the 1930's, if I recall correctly). And when the judo man tried to grab him, the Kiraku-ryu man put on an "ara-uma" - (literally - "wild horse" - which was a nikyo).
2. Takeda Sokaku was riding on a train and a big, ill-tempered American named Charles Perry tried either to throw him out of the 1st class car, or have him thrown out, not believing that a shabbily dressed little Japanese guy had a right to be there. Takeda slapped a nikajo (nikkyo) on him. Perry ended up studying a little bit, the first non-Japanese to study Daito-ryu (which could be a whole other discussion - Takeda was so at home with himself in one respect, that he was willing to teach a foreigner in Meiji Japan!)
3. Takeda also went to pay a visit to Ueshiba in 1936. Nakakura Kiyoshi, if I recall correctly, ran outside at the sound of howling. Takeda had a taxi or pedicab driver in a nikajo, thinking he was overcharged. When that was sorted out, he asked where Ueshiba was. When told Osaka, Takeda turned around, and went to the Asahi Newspaper - the rest is history (and there was Ueshiba, dimed out by his own son-in-law).
4. And in reading the story of Shirata sensei, part II - Aikido Journal - we read that Shirata tore the ligaments of a challenger, a very big judo man - using nikajyo.
It is really fascinating to me that in all these occasions - and I'm sure others - the almighty victorious Daito-ryu/aikido technique - - - - -- - is nikkyo.
I've seen the same thing with t'ai chi - a variant of nikkyo is one of their go-to techniques.
One thought is this. This is why MMA is so important! Most modern martial arts, in making it sports or budo, circumscribe the techniques used. If you don't practice something, it can disappear as an option. So when attacked with such a technique, it's a total surprise. It doesn't take a judoka long, if he or she "admits" the possibility of wrist-locks, for a nikkyo to be impossible to apply. BUt one has to be aware. (Similarly, kyokushin-kai karateka didn't practice blows to the face - only kicks. When they came to the muay thai gym I trained, invariably, they got lumped up - they forgot how to protect their face).
I meander, perhaps. I simply struck me, though, about nikkyo. Over and over again, I read about a Daito-ryu, an aikido or a jujutsu victory, and nikkyo is, I think, the most common victory technique.