Kuzushi
I train in an offshoot from Tomiki, so I'm not intimately familliar with the specific techniques you're asking about... if I'm correct, what you are describing is contained in the Tanto Dori section of Koryu Dai San kata. Just checking to see if I'm on the right page with you...
in any case, (IMHO) at the first touch Tori (Nage) should cause kuzushi [complete distablization of the musculoskeletal frame) making it nearly impossible for Uke to make an "effective" attack without first recovering their balance/stability. Now, granted, it doesn't take much to do some nasty damage with a knife, but if kuzushi is effectively made, the automatic reflex of the body (involuntary reflex) should disrupt the attack (by range, positioning, arm reflex, etc depending on the situation) for the time in which it takes Tori to effectively move to a better position.
However, many schools have lost the biomechanical understanding of what kuzushi is and how it works, so their waza becomes ineffectual. It's all very dependent on how their instructors have learned/taught.
On the other hand, in my experience Tomiki groups in general have a poor understanding of "good knife technique" (my group has been trying to improve our understanding over the past 10-ish years), and if you only know how to give poor attacks, you can only defend against poor attacks. It's possible that the waza is functionally lacking and needs to be redesigned.
Or, it could be a worst case scenerio. I've encountered several waza in the Tomiki corriculum which I think "I wouldn't do that!!!" but it's there because "what if the situation came up and that's ALL you had?" That's pretty much the attitude you have to take with the Tachi Dori section of Koryu Dai San... If a person with the sword knows what their doing and seriously wants to kill you, those techniques have maybe a 1 in 10,000 chance of working, but if you've got the choice between Dead or 1/10,000 Chance-of-Living, which will you take?
Matthew White
Jiyushinkai Aikibudo
Oklahoma City, OK