When we first begin Aikido, we train with a clear distinction in our minds between the roles of uke and nage. Training is deliberate and choreographed and we know that uke always attacks and takes falls, while nage always throws. Over time, uke becomes sensitized to openings and weaknesses in nage's technique and is aware of opportunities to strike or reverse the throw. This type of training is formalized in the practice of kaeshi waza. There is no longer a predictable outcome to techniques since either partner can become uke or nage as the throw unfolds, and the fall itself potentially becomes a throw. As a result, our concept of practice becomes less rigid and we do not so much see ourselves as an uke or a nage, but become more involved in the process of sensing the movement and intent of our partner.
This awareness of our partner is an evolving process and, for most of us, has to start at a very basic level. At first, we are only conscious of the most obvious elements of our partners movements and even then only when we are assured that uke will stay in their role as attacker, follow our lead, and eventually take a fall. At this stage of training, attempts by uke to reverse the throw often result in nage becoming tense and focussed on throwing uke down by any means possible, rather than developing an awareness of how uke is responding. Kaeshi waza is not a useful training method at this point since we are only able to keep our awareness extended when uke follows the throw without resistance.
With additional training, our urge to struggle against attempted reversals lessens and we become more receptive to uke's movements. It becomes psychologically easier to accept throws that don't go as planned and to make adjustments in mid - technique. Awareness has developed to the point that it is not automatically pulled back in when conflict arises, although relapses are all too common. Kaeshi waza now becomes an interesting and useful practice as the two partners are able to ride with the ebb and flow of the technique(s). At highly advanced levels, the kaeshi waza following an initial attack can continue indefinitely and it becomes impossible for an onlooker, or the partners themselves, to distinguish between uke and nage.
Chris Swenson, July 10th, 1992, Aikido North