David Alexander Sensei spent over 10 years training in Iwama under Saito Sensei, in the old days when understanding was obtained from vigorous and rigorous practice. In an article he explains the four levels of technique in Aikido training:
1. KATAI (rigid)
Also known as Kihon (basic) it is what builds the foundation of tai-sabaki (body movement) and kokyu-rokyu (abdominal breath power).
2. YAWARAKAI (flexible)
Like bamboo bending in the wind,the Yawarakai level emphasises the principle of awase and requires the defender to give in resiliently to deflect the attack in a direction that he can merge into the movement and take control. Yawarakai technique is intermediate between Katai and Ki-no-nagare.
3. KI-NO-NAGARE (flowing)
In Ki-no-nagare technique, the defender does not wait for the attacker to obtain a grip, but begins merging into the attacker’s movement before contact is made.
4. KI (spirit)
Saito-Sensei explained in his book, Traditional Aikido Vol. 5, on page 36 that: “Aikido is generally believed to represent circular movements. Contrary to such belief, however, Aikido, in its true Ki form, is a fierce art piercing straight through the center of opposition”.
For Alexander Sensei’s full article see Levels of Technique in Aikido Training.