Re: knife defenses
I liked the advice about the three Cs of dealing with a knife (Clear, Control, and Counter). I would like to add that the number one thing we should be working on is maintaining a safe distance (angle, etc.) so that you could possibly clear, control, and/or counter. Maybe that's obvious, but I see so many people in terrible position trying to force techniques that it's become a pet-peeve. I generally cannot lift a person if I'm at arms length from them - and every technique basically works the same way. That's just aikido in general.
When we specicially work with taking a knife, I'd say that if you do have to offer them something you want it to be the back of your arms rather than the other side. When you do find yourself trying to control that blade, you want to grab and maintain control of their opposing digit. If you control the thumb, you can strip that knife. I see a lot of tanto dori where the nage grabs the wrist. That is a poor choice if you could have gotten their thumb. It takes a bit of re-training but it's well worth it. I'm curious what the folks who do tanto kata think about this.