N. Tamura in Aikido quotes four methods of training through which the aikidoka progresses to eventually achieve a state of perfection called ki-musubi.
Trans: solid body.
Ko-tai is the first level of work, it is solid an precise and builds the foundations of all other training.
During ko-tai training, tori allows uke to take a firm grasp and both partners assume strong stances. In this level of work the techniques can be examined in detail and any necessary corrections made to their practice.
Uke is required to give resistance to tori so that tori will learn. This does not mean that uke fights to resist tori's techniques but more that uke will only fall if tori succeeds in breaking his/her balance.
It is said that the bulk of the work up to and including shodan should be ko-tai in order to build a firm foundation for the practice which is to follow.
Trans: fluid body.
Tori moves as uke makes contact. Ju-tai does not imply thatuke's attack is soft or without substance.
There is a tendency when practising in a ju-tai mode for the practice to get faster and faster. Remember that fluid does not necessarily mean fast. Ju-tai practice should always be based on ko-tai practice to ensure a firm foundation is not lost.
Trans: liquid body
At the moment uke intends to act, tori begins to move.
This is a high level of practice where tori's breathing is synchronized with uke's movements. It is really only applicable to senior dan grades.
Trans: spirit body
We're into the realms of very high-order aikido here.
I don't believe I am qualified to try and explain this level of practice so I refer you to Tamura sensei's Aikido.
An attacker strikes and there is no longer anyone there.
This movement implies the action of ki. It is an exercise of ki-awase (becoming acquainted with ki.
It consists of enveloping and engulfing like a fog which envelopes and swamps everything.
Guiding aite in the way he wants him to go it is no longer necessary to touch him.
I think that O Sensei was describing this state when he said:
I have only to remain standing, silent.
It is a state of perfection called ki-musubi.