Entering, or "irimi" is one of
the basic techniques of Aikido and is closely related to "blending" with
an attacker. At a basic level, irimi is a movement which looks like a
slidestep toward or into an opponent's attack. As mentioned before,
Aikido thinks of most movement as being circular or
spiralling in nature; irimi brings a person "into" the circle of movement,
so that the energy of the attack can be directed along the circular plane.
The concept of entering
emphasises the importance of placing oneself inside the "danger radius" of
a partner's attack. Imagine a boxer's punch. The punch has gathered most
of its power and effectiveness at or near the full extent of the boxer's
arm. Beyond the reach of the arm there is little danger or threat.
Similarly, inside the full
extent of the arm the moving fist has developed very little energy, and
again poses little or no threat. The Aikido practitioner grabs the
attacking limb while at the same time entering this “danger radius”
excellent book - Aikido, by Kishomaru Ueshiba.