I discussed this on another thread some time ago (#15849646 ) and gave "non-kyusho" instructions about doing yonkyo. I don't like to "cross over" the discussion between hitting Points x and y and aikido technique because they are done off differing theories of execution.
Where the kyusho version of it is hitting metal and fire points simultaneously, the idea of aikido application is extending ki into the radial nerve. If you "support the elbow", it traps the energy at the point of application and gives the uke the heartwarming feeling that brings tears to their eyes...

I love yonkyo... it's my favorite nerve technique... that's where I got my nickname "Dr. Pain"

If you follow the instructions in that previous thread, you can squeeze your forefinger and thumb against the wrist and do a perfect "kyusho" technique (especially at the heart meridian side) of the wrist.

Squeeze slowly... and enjoy...
