I had to go beyond the dojo I was in the learn the yonkyo pin corectly. Sadly on my early kyu exams, we just had to fake the pin or try to get a stretch to make uke tap. Yonkyo is a tricky thing and No one will get it nless they have been instructed by somone really advanced. My nidan sensei at the time did not even know how.
Now, I have been taught by a couple aikido greats and it never fails. Bryan is basically right alothough there are a few more points to consider. I will apply the usual nerve pin only once which usually works and if not, I waste no time by going into something that will never fail! Maybe I can explain later. Gotta run for now.