Mastering simultaneous leading and following uke is important if I wish to practice Aikido techniques with a minimum amount of invasive intrusion into uke's space and little or no control of uke's movement. On the surface it appears that minimally invading uke's space and not controlling uke's motion run counter to the idea that the object of executing Aikido technique is to absolutely control every aspect of the encounter with uke.
For me to willingly forgo the notion of control I have to accept the fact that, once entered into, an interaction with my partner will assume any of a wide variety of forms. Even within structured technique practice with the same partner each iteration will differ from all the rest in some respect. Motion vectors, speed, intensity, intent, connection; are some of the variables that will have different values with each round of execution. If I'm not going to control uke how then can I successfully execute technique?
Simultaneous leading and following enables me to achieve what I refer to as congruent motion with my partner. When we have congruent motion our movement is in perfect harmony where there are no opposing forces to block, deflect or otherwise deal with. Since I'm not doing anything to uke in order to get him to do what I want, he has nothing to resist against. Our trajectories coincide and the "our" disappears, subsumed by connection. With the disappearance of "our" so too does the need for control vanish. The technique and its logical conclusion grow out of the motion naturally. Techniques that grow out of congruent motion feel effortless; touch is kept to a minimum yet uke is taken to the mat as inexorably as if a mountain had fallen on him. For me, this is the essence of "soft" Aikido.
(Original blog post may be found here