Keep weight underside
When students keep weight underside, they release all tension and rigidity. They are relaxed, with a low center of gravity. A stiff upper body results in a lack of balance and breathing that is "caught." Weight underside results in a stable posture and increased responsiveness on the mat.
Just as a bow kept strung loses its usefulness, so humans cannot stand continuous tension. It is essential to be able to relax both body and spirit, as needs demand, in any place at all. Many people feel that it is impossible to relax when something big is happening. This notion arises from the illusion that when one is relaxed he is weak. The fact is, if you relax properly you are very strong.
Keep one point
When we perform Aikido techniques, we always maintain the single spot in the lower abdomen, focusing our mind on a single point and maintaining a strong center of gravity. It is not necessary to tense any part of your body to "keep one point" - relaxing will result in a more stable posture with no mental distractions.
Ki is energy, spirit. Our Ki is in conflux with the Ki of the universal. If we pour forth as much Ki as possible we can improve this interchange. We can pour forth all the Ki we like because the supply is inexhaustible. Once we have made our Ki positive we should not be satisfied to stop. Whether we make something brilliant of our lives or not depends on whether we select the positive or negative way. Aikido training helps us create and extend positive Ki.
Adapted from Aikido in Daily Life, by Koichi Tohei, Rikugei Publishing House, Tokyo, Japan, 1966.