Re: Anybody know what this technique is called?
We refer to it as simply "Tanto dori kokyunage" - along with a number of other techniques. Informally we often describe it as a reverse headlock and it is a staple in our tanto dori techniques. We take it to the mat and kneel with our outside leg down and inside leg up while facing towards Uke's feet. We pin by grasping Uke's knife arm across the raised knee and grasping the back of his neck. By raising on his neck and pushing down on the arm as we lower our knee, Uke flops over on his belly and we pin similarly to ikkyo to take the knife away.