Many years ago I studied aikido
It was part of training in movement that I undertook for a poor theatre troupe and workshop, along with many other forms of movement, including dance and mime.
It must have been much like amelinda describes: redirecting your opponent's energy. Our sensei in the dojo, and our masters in the troupe, spoke about the ki, I think it was, basically the solar plexus, or a chakra.
The idea seemed to be to absorb, and redirect the motion of all actions. as a consequence, being thrown by the sensei was quite an experience. From standing, he would take my arm, then, with hardly a breath--by him or me--I would be turning, head over heels on the mat, then, with just enough force, I'd come to a standing stop at the other end of the mat. No one else could manage. Its a real bore to run out of steam flat on your back!!
I had two problems, which quickly ended my short career:
I wear glasses. Just as they make swimming, and especially taking lessons in a pool difficult; in the dojo, it was near impossible. I couldn't really wear them while being thrown, or interacting with others. I'd have to put them on to watch demonstrations, then remove them for practice.
And the dust! This was the killer. I couldn't breath! The tickling in my throat was unbearable.
Fortunately, the Idea of movement that my masters wanted to impart I picked up from the other Practices.