Aikido techniques of uke and nage
Aikido training is based primarily on two partners practicing pre-arranged forms (kata) rather than freestyle practice. The basic pattern is for the receiver of the technique (uke) to initiate an attack against the thrower (投げ nage, also referred to as 取り tori, or 仕手 shite, depending on aikido style), who neutralises this attack with an aikido technique.[24]
Both halves of the technique, that of uke and that of nage, are considered essential to aikido training.[24] Both are studying aikido principles of blending and adaptation. Nage learns to blend with and control attacking energy, while uke learns to become calm and flexible in the disadvantageous, off-balance positions in which nage places them. This "receiving" of the technique is called ukemi.[24] Uke continuously seeks to regain balance and cover vulnerabilities (e.g., an exposed side), while nage uses position and timing to keep uke off-balance and vulnerable. In more advanced training, uke will sometimes apply reversal techniques (返し技, kaeshi-waza?) to regain balance and pin or throw nage.
Ukemi (受身, Ukemi?) refers to the act of receiving a technique. Good ukemi involves a parry or breakfall that is used to avoid pain or injury.[24] In applying a technique, it is the responsibility of nage to prevent injury to uke by employing a speed and force of application that is commensurate with their partner's proficiency in ukemi.[24] Injuries (especially those to the joints), when they do occur in aikido, are often the result of nage misjudging the ability of uke to receive the throw or pin
Source : http://en.wikipedia.org/wiki/Aikido