As students advance, an important element of their practice becomes unbalancing their uke. This is referred to as kuzushi – a Judo term- which derives from the Japanese verb, kuzusu, meaning to level, pull down, or demolish. Kuzushi includes initially unbalancing the attacker and then continuing so that the attacker is unable to regain balance.
This unbalancing ensures that Aikido can be effectively practiced regardless of the difference in size and stature (e.g., big, strong attacker vs. a small, lightweight nage). As you continue to practice, you develop a sense of the subtleties of kuzushi and begin to experience that a small unbalancing is all that is needed. But, you come to learn this through the feel of taking uke’s balance by bigger movements.
One way to create kuzushi is by entering (irimi), reflecting back the energy of uke’s strike into an area of instability behind him/her. The irimi entry needs to be immediate and unhesitating. Shomen uchi ikkyo is an example of this.
It is also possible to draw the attack forward into imbalance by stepping beside the attacker (tenkan or irimi) and drawing the stability away from the back heel of the attacker. Stepping back (tenshin movement) nage can also unstick the attacker from the stability of the back heel.
Atemi (strikes against uke’s body that the attack exposes him/her to) in the Aikido encounter can also create imbalance, pausing uke’s attack. It is best, however, to learn to unbalance uke right from the start of the encounter and to lead the attacker so he/she does not regain balance.