I dropped in at Bozankan and Jim was teaching.  Starting from gyaku hanmi, Uke steps in with the tantou in an overhead shomenuchi strike.  The front leg has to step slightly off the line of attack.  Then, unlike ikkyo omote, the back leg has to step in while both hands come up and grasp Nage’s knife forearm and wrist. Nage’s hips rotate and continue as if an ikkyo pin. Except, because Uke is holding a kinfe, Nage has to slide out 45 degrees and straight to the ground in the direction Uke is facing.  Any delay or slack gives Uke a chance to cut Nage.