02d Storing Energy

There are 2 ways to store energy for later release. One is potential energy and one is as energy stored in a spring. Traditionally this might be interpreted as heaven and earth or tench.

When energy is applied to a human either of these mechanisms might be employed, applied energy might take the form of a push or the jerk of a bus or subway train as it comes to a stop.
Heaven (Potential Energy)
When pushed or having to stop suddenly often we rise up on our toes and arms might come out to our side, its a terrific way to stop suddenly and often momentarily. To move again we drop our weight and convert the energy into motion again. The only trouble with rising up is that stability is sacrificed.
In the aiki arts we can make use of this process, to raise up the centre of Uke and put them on their toes, then with controlled release (NOTE: not throw) we can redirect this energy, the effect of which is the Uke is thrown - though not as a throw

Earth (Adsorbing Spring Storage)
Runners make use of 'leg stiffness' to bounce of the ground and keep moving forward, its an efficient way to store and release energy - though will eventually fatigue muscles. In the same way you can drop your centre to adsorb applied power into your body or into the ground. To release this power the spring of the body or pushing off from the ground (this requires effort) can return it from wench it was sent.

In the aiki arts adsorbing into the skeletal frame, tendons and fascia of the body and allowing the 'spring' effect of structure to return the power to Uke. With correct timing this achieves Kuzushi. A a more macro level (i.e. you can see more easily that the former), dropping the centre quite a bit to enhance stability - whilst drawing the centre from Uke can then allow the rising of the centre (pushing against the ground) to release the energy. This is not really energy storage, but since the body can't raise its height indefinitely the temporary lowering of the centre then gives it room to move in the upward direction.