A while back, Nick came up with a scheme for organizing our aikido explorations - he said that he divides the whole domain of material that we can practice in aikido into three circles - foundational, supplemental, and transformative.
The foundational material is the stuff that we think every student should work on first until they become quite competent with it - material like ukemi, taisabaki, releases, Junana, Owaza - this is most of the focus of most of our clubs up to about shodan level.
The supplemental material adds to the foundational, expanding its scope or applying it to specific contexts. This would include material such as the Koryu no kata, tactical systems like Merrit Stevens' or JW Bode's, and aiki weapons systems - in other words, stuff that we mostly work on from about nidan up.
The tranformational circle of practice would be various practices that re-interpret or re-contextualize all of the foundational and supplemental material. Anything that magnifies the aikidoka's capacities by fundamentally re-imagining or re-examining the previous material. Such material would include randori, the renzoku chains, and our explorations of internal strength and systema. Transformational material is not linked to a skill level like the other two circles of practice. For instance, one starts almost from day-one transforming their understanding with randori.
I personally think that the addition of the knife to our practice is more transformational than it is supplemental. The knife is just that good of a training tool. What the knife does for our practice is nothing short of transformational. We are not adding x-number of knife defenses to our regularly scheduled (foundational) practice. We are using the knife to change the parameters of our foundational practice.
It is quite easy, when practicing empty-handed aikido - especially because we emphasize slow and gentle - to develop the illusion that uke is not really a threat. Becoming comfortable dealing with a slow, gentle, compliant uke is probably the deadliest error in aikido practice - it creates an intolerable laxity in our practice. But throw a knife into the mix and that illusion is dispelled instantly.
The addition of the knife sharpens uke's attacks and it simultaneously hones tori's concentration and awareness. It forces tori to start working in the domain of strategy as opposed to technique or tactics.
Because of the transformational nature of the knife, we have recently been adding practice knives to all of our foundational practices. We do the walk holding a knife in one hand (totally transforms your perceptions of which hand is the active hand at any given time, and it tends to make you more aware of where your hands are and their orientation.) We do the releases with uke holding a knife in his free hand. Even without uke overtly lunging with the knife, just its presence is enough to create a big perceptual difference in the exercise. We do all of our junana practice with uke lunging with a knife, as in the tanto-ni-taisuru kihon no kata. We haven't been working Owaza with a knife - but just because we havent had anyone working Owaza for a while. I fully expect to be playing knife Owaza soon.
As one of my FMA buddies was telling me some years back. "The knife teaches the hand, but the hand does not teach the knife." If you don't understand that idea, I recommend adding a knife into your regular practices and looking at how your practices are transformed and sharpened.
The addition of the tanto as a training tool is, in my opinion, one of Tomiki's greatest contributions to the practice and teaching of aikido.
Incidently, as a sort of postscript... I have the feeling that our jo and bokken material should be transformational instead of supplemental. It is my understanding that this is what made such a distinctive difference in Nishio sensei's remarkable budo - the inclusion of the bokken into all of their foundational practices. But the way we tend to practice jo and bokken (as an aside as part of a couple of of the koryu no kata) it is supplemental rather than transformative. I would like to change that - but first things first - grab a knife.