Nafis Zahir wrote:
... He showed me how to gain control and cut the person down by using the "sword cutting version" first.
In addition, I might add, you really need to do a lot of Bokken cuts and "ringing" the Bokken handle at the end of your cut. This will give power to the point on your hand that actually applies pressure to the radius bones. People who don't practice weapons generally do not have proper grip for this technique.
I have only ever seen this Yonkyo type technique in Aikido, and in my opinion it comes from sword training ... almost as if someone with a strong sword cutting technique once tried it on an arm and got good results . "Hey this works!".
Sword cutting practice can give surprising strength for this one technique.
(Caveat : I practice cutting, but I'd still avoid trying Yonkyo in a real fight : I just don't trust it to work)