A concept I've been thinking about for a while would be the concept of " The Cilinder". This is a description of a principle which concerns itself with the position of uke in relation to tori. When practicing the Zhan Zhuang posture the cilinder is identified as the space which is in front of tori when the arms are held in the hugging a tree posture.
Not like this, but more like this...
So it's a virtual cilinder in front of tori. I'll call the space in which you have the most control of uke inside the cilinder, if uke touches the outside of the cilinder,it is called outside the cilinder.
What is the relation of all this to our Aikido practice?
When training in Aikido uke is usually held outside the cilinder for protection of oneself and defensive in nature. When executing a technique, uke is allowed to enter the cilinder, and therefore easily controlled.
In the next clip this is easily studied. Irimi nage is an excellent technique in which this concept is identified.
Try to incorporate these concepts in one's training and enjoy practice.