The following is meant as a discussion of some possible meanings for the five principles and is not meant to limit the possible definitions for any given principle.
Extending ki is also one of the Four Principles of Mind and Body Unification. Ki extension is a principle of the mind which affects the body. The mind leads the body, and ki extension is the connecting link between the thought in the mind and the action of the body.
Another way of saying "extend ki" is to say "extend the mind". Many aikidoka extend their arms physically, but lack the unity of the mind in performing the motion. This is "extending limbs", not "extending ki".
Master Koichi Tohei, founder of Shin Shin Toitsu Aikido, offers the parallel rule, "Know your opponent's mind."
The word aikido is composed of three Japanese characters: ai (harmony, ki (energy), and do (the way). One possible translation might be simply the way of harmony with energy. In the performance of the arts of aikido, the energy of the nage (defender) must harmonize with the energy of the uke (attacker). Before one can be in harmony with the energy of another, (s)he must first determine the intent of the attacker.
This intention can be broken down into two components: the intention of the mind and the intention of the body. Before commencing an attack, uke will have a certain mental attitude. If this attitude culminates in an actual attack, the attack itself will have certain characteristics in terms of direction and intensity. To successfully defend oneself, the aikidoka must understand both the mental and physical components of uke's attack, and this understanding must begin with uke's mind. By being sensitive to uke's mental and physical states, nage will become aware of the exact intentions of uke, and application of the proper technique will be naturally forthcoming.
When a student first begins studying aikido, (s)he will often hear senior students and sinstructors using the phrase, "Lead the opponent's mind; his body will follow." This is only possible if one first understands the opponent's mind.
In their book Aikido and the Dynamic Sphere, Oscar Ratti and Adele Westbrook define the motions of uke and nage in the early stages of an attack as motions of convergence. The goal of nage is to move in harmony with the energy of uke, guiding it into a circuit of neutralization, a movment which by its very nature not only dissipates the energy of the attack harmlessly, but which succeeds because of its non-dissension with the attacking energy. Such a movement is not possible if one does not recognize uke's ki and respect it.
By putting yourself in someone's place, you develop a different understanding of his or her perspective and motivations. Such an understanding may allow you to defuse a potentially explosive situation, eliminating the need for a physical altercation.
If an attack does occur, the performance of an aikido technique typically involves substituting nage's center (one point) for uke's center. Wheras uke initiates the motion of the attack, nage performs a technique in which (s)he becomes the center of motion, leading uke's energy in the process. Mune-tsuki kote-gaeshi tenkan is a good example of the substitution of nage's cetner for uke's.
Will Reed, in Ki: A Practical Guide for Westerners, says it best: "Do the thing in your mind quickly, and relax completely knowing that it is done."