What is harmony? Harmony is one of those buzz words that all Aikidoists talk about. Harmony is when two or more things come together. In an Aikido sense, harmony can be achieved energetically or physically when we align our power with our partners. (As a side note: we also harmonize with our partners mentally and emotionally but that is more complicated and something for a different day). Harmony, for lack of a better explanation, is nothing more than alignment. We align with our partner's body on an energetic level or ki (氣) level and on a mechanical or physical level. Throughout the technique, Aikidoist use this idea of alignment in various ways.
At the initial onset of attack when we are engaged by an opponent, the first step for us to do is to neutralize the attack and we do this with alignment. Generally speaking the first step in any Aikido technique is either irimi (moving forward) or tenshin (moving back). Both of those movements allow us to move "with" our opponents power and thus neutralize their attack.
Once the attack has been neutralized we then create alignment again in a different way and begin to use their power. This is where the idea of "matching" comes in. Matching happens in almost every Aikido technique. If one looks closely at the technique, they might see instances where we are aligned with our opponents. This alignment can look like our hips, feet or body position going in the same direction as our partner. This alignment allows us to bring their power into ours but never ours into their power. Take for instance tenkan, after the initial irimi and tenkan movement our bodies are essentially going in the same direction as our opponents (see picture). Once we create alignment with their power, we can then use their power for our needs which is that we usually redirect it and throw it or pin it.
What type of "power" we align with depends on our training. The less experienced have a hard time with energy and need to rely on the physical while the more experienced don't need to the physical and can use the energy or ki more.
Therefore, harmony is nothing more than alignment. When we can align with them we can see where they are coming from and thus take them where they need to go. Once we are aligned with their power we can then use it and thus the Aikido techniques are born.