In Iwama we learned of the notion of Yamabiko, the mountain echo, that refers to the natural response in the blending in Aikido. In the dojo we train this interaction. We "call out" and receive an answer. We initiate and blend with the response. Instead of waiting, for example, for a Shomen (frontal attack) (like in the old days or like in Daito Ryu) O Sensei changed it to become pro-active. So he would initiate the Shomen movement to bring out the response from the opponent. In this way we are able to stay with the opponent from early on, taking the initiative away from the attacker. This understanding is crucial in practicing awase, or blending in Aikido.
It is a natural response just as when you reach out to shake someone's hand, they respond in kind almost automatically. It is an inherent, pre-conscious reflex of the body. Someone smiles at you and you smile back. Someone puts a fist in your face and you pull back or raise your hands.
There's a wonderful secret in this that is deeply human and spiritual. It's all about a relationship and being alive to the world around us. Once we are keenly aware we'll be able to read the situation and respond or initiate appropriately. Once we're fully awake this will come naturally to us. A perfect response in any situation. The event lending itself to a match that completes and fulfills.
Yamabiko reflects the harmony of the world in detail. It is ultimately a deeply spiritual perspective and a perfect way to engage with others.
As Russ offered, I use this concept a lot. It functions at all levels of technique. At a distance it governs what happens when you project your intent over distance and connect with the partner's attention.
It also functions when you actually make contact. In the west we call this principle a "conditioned response", but I really like the visualization of he mountain echo. What does an echo do? The sound goes out and hits something and bonces back, hence the echo. When you connect with a partner on the physical level, that's exactly what happens. You touch their center with your intent and then receive the energy of that connection back to your center creating an energetically balanced contact point thereby neutralizing the opponent's strength. This is Ushiro Sensei's "zero power".
Eventually, technique can move from the physical towards the energetic in which the touch no longer is physical but rather the touch is to the mind via the perception (the sensory inputs). You move the mind so the mind moves the body.
The "out and return" principle is one of the most important principles in Aikido and yet you don't hear many people talk about it. I first encountered it in Saito Sensei's books back in the seventies but didn't have the background to understand what he was talking about. Now I do. I don't know if it precisely the same way Saito understood the concept... I never had a chance to work with him. But I find the concept to be crucial to what I am doing in my Aikido at this point and it seems to be very helpful in helping others to improve.