Once the basics are covered (a process which, unlike other martial arts, really starts at Shodan (Blackbelt) in Aikido) there are two concepts which are crucial for the development of higher level Aikido —Irimi and Tenkan.
Irimi(入り身)(ee-ree-mee) involves entering into an attack instead of standing there to receive it or stepping backward to block or defend it. To the uninitiated it appears that the Aikidoka is evading the attack. While not being hit has a number of benefits for your defense, what Irimi really represents is the opportunity to blend with an opponent’s attack, leaving the opponent with nothing to strike. Instinct and experience teach us to freeze or slightly better, to move back from a punch or a kick. Instinct never tells us to move into an attack. The very notion of doing so is so counter intuitive to our experience. And that is why it is so effective. The timing of Irimi is also missed at lower levels of training. Irimi movement is most effective if it occurs at the same time of your opponent’s attack. As in Kendo, the attack creates your opening to move and counter. Your attacker is expecting you to be where he is striking, not coming towards them. This combination of an opponent’s attacking momentum met with forward movement allows for great speed and ‘surprising’ force.
Tenkan (転換) (ten-kahn) is a turn or 180 degree pivot to the side or rear of the attacker. Depending upon which foot is forward in your hamni, the pivot is either clockwise or counter-clockwise. Again, it looks a lot like flight and in only the strictest sense, it is. A rapid turn to the “blind” side of your opponent places you at an advantage for executing defensive techniques, such as: shihōnage (4 direction throw); kokyūnage (timing throw); and, atemi (counter-strike). Further, the turn itself allows you to blend with the attack and translate the forward movement of your opponent into rotational force generating power from your hips. As with Irimi, Tenkan movement is most effective if it occurs at the same time of your opponent’s attack. Admittedly this can be a tall order but the longer you train, the quicker your timing (and your turns) will become.
“Yin and Yang” describes how seemingly contrary forces are actually interconnected and interdependent as well as how they give rise to each other in turn. In Taoist philosphy the simple act of dropping a stone into a calm pool of water will cause a disturbance in the water with waves radiating outward until the movement dissipates and the pool is calm again. To remember the concept for the practice of Aikido, Irimi -the motion of entering may be thought of as Yin, to the turning motion of Tenkan -Yang. Thus, practicing Irimi and Tenkan movements help us develop balance (Hara), not only physically, but also thoughtfully.
It is not expected that this concept will take until one has spent many hours and years on their practice. Such a seemingly small set of movements, however, capture what is unique about the martial art of Aikido.