The art of  irimi (entering) is utilized during the instant of your opponent’s attack. It means to move out of his line of attack to his shikaku or “blind spot” In Aikido if your movement is correct, you become similar to a spinning top. At all times you try to get your opponent involved in turning against your center axis. This movement must be positive, spherical and smooth, no matter what force attempts to interfere with it.  Through TAI - SABAKI (body movement) you get your opponent into your spherical movement and guide him at will, like a spinning top throws off things which touch it. 

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”

Source: An excellent book - Aikido, by Kishomaru Ueshiba.