Motobu Choki shows this application in his 1926 book on "kumite" applications. Although originally from the Tomari lineage Passai kata that both Motobu and Kyan knew and taught, it can easily be extrapolated to Sunsu. Due to this, it is my conviction that Tatsuo Sensei took this part of Sunsu from the Passai that (I believe) he learned under Kyan Chotoku.
(Fig.1) As the opponent strikes to our face with his left fist, we use our left hand to strike his arm off its course, and immediately grab his left wrist with our left hand.
(Fig.2) After blocking the opponent's left attack with our left hand, then twisting our left hand top grab his left wrist, as in the previous diagram, we grab his left elbow with our right hand, and
stomp on his knee with our right foot to break it.
PS, I must thank Victor Smith for originally bringing this to my attention after he had met with Sherman Harrill a couple of years ago...
By Joe-Swift 9/4/2001
Note: It was Harrill Sensei who pointed this out to me at a clinic in Rhode Island, when I shared this with Joe-san he performed the translation and sent a copy of a Japanese edition of the Motobu book to give to Harrill Sensei in turn.