Extension, expansion, and relaxation are closely linked to each other. Extended movements are relaxed, with the weight naturally below, and centrifugally directed (from the center outward). This means that the ultimate direction of your movement will be out your fingertips and toes, as well as out the top of your head and the base of your spine. It means that you continually expand in your training, that your presence enlarges, and eventually fills infinite space. Such was the experience of O'Sensei, aikido's founder, of "being one with the universe".
The unbendable arm exercise is possibly the best way to start learning extension through the arm. If you haven't experienced it, get someone in the dojo to teach you. Extension can also be practiced through your elbow, knee, shoulder, or any other part of your body.
The idea of any extension exercise is to make your body bigger, to gain more presence in space. extension is expansion of ki, mind, and body. This does not mean that you will get fat, but rather that you will fill all the space to which you are entitled. As your ideas about yourself become more positive, you will be able to enjoy new extensions of your being every day.