If you look at the different forms of energy practice, they differ in posture, in visualization and in many other things. What they all share is the breath. In Qigong, energy is called qi 氣, and in Japan the same character is read ki. Reiki carries the character in its name, rei-ki 靈氣, but there the rei 靈 defines the kind of ki: spiritual life energy. So it is worth taking a closer look at the breath.
In East Asian traditions breathing is much more than a way to relax. It is one of the most important means of gathering and guiding ki, and that makes it a good key to understanding the connection between Qigong and Reiki.

Belly breathing 腹式呼吸
In Japanese, natural belly breathing is called fukushiki kokyū 腹式呼吸, literally "belly-style breathing". It is the way every person breathes when they are born. If you watch a sleeping baby, you can see the belly rising and falling while the chest stays calm.
Over the years many people lose this natural way of breathing. Stress, tension and a lot of sitting move the breath up into the chest, where it becomes shallow. That is why coming back to belly breathing is the starting point of so many practices, in Qigong as well as in Reiki and meditation. In Reiki, for example, it is the basis of jōshin kokyū hō, the breathing method for purifying the mind.
There are also advanced forms, such as reverse belly breathing. Forms like this are best experienced with someone who has practiced them for a long time, which is why I only mention them here.
Hara and tanden 丹田
So where exactly do you breathe when you breathe "into the belly"? Japanese and Chinese traditions give a precise answer: into the tanden 丹田. The two characters mean "cinnabar" or "elixir" and "field", so the tanden is the "elixir field". In Chinese the same characters are read dantian. In Japan this center is also called hara 腹, which simply means belly. It lies about two finger-widths below the navel and further inward, and it shouldn't be confused with the sacral chakra, which sits lower.
The tanden is not an organ in the anatomical sense, but an energetic center. In Japan people say of someone who is calm and grounded hara ga suwatte iru 腹が据わっている, "his belly sits firmly". It is no coincidence that martial arts, the tea ceremony, calligraphy and meditation in Japan all start from the hara.

For Reiki there is a very interesting point here, which I describe in my book on Reiki in therapeutic practice. Before his experience in the Kurama mountains, Usui apparently knew methods in which you first have to gather ki in the hara before you can pass it on. He was looking for a way to work without this gathering first. Even so, the hara remains an important center in Shingon Reiki, for example as a place of focus in meditation and for centering yourself. In the article on meditation posture you can read why the way you sit matters for this.
Breathing through the nose and the 108 breath meditation 數息觀
A question that comes up again and again is whether to breathe through the nose or the mouth. It depends on the tradition. In Qigong there are exercises in which you breathe out through the mouth, for example with certain sounds. In Japanese Zen and in Shingon, people mostly breathe in and out through the nose. That slows the breath down by itself and supports a quieter attention.
This calm breathing through the nose is also the basis of sūsokukan 數息觀, the meditation of counting the breath. It sounds very simple, but anyone who has tried it knows how quickly the mind wanders off. The 108 breath meditation is one of the oldest breath practices in Buddhism, and it is part of Shingon Reiki and Kuji Kiri.
The number 108 appears again and again in Buddhism. A Buddhist rosary (jap.: juzu 数珠) traditionally has 108 beads, Buddhism speaks of 108 worldly desires (jap.: bonnō), and on New Year's Eve the bells of Japanese temples are struck 108 times.
Standing meditation 站樁功
In Qigong there is a practice that looks like nothing at all from the outside, the standing meditation, in Chinese zhan zhuang 站樁功. The name means roughly "standing like a post". You stand still in a relaxed posture and breathe, sometimes for a few minutes and sometimes for half an hour or longer.
What makes standing meditation special is the combination of posture and breath. When you sit, the breath can be deep, but the contact with the ground is different. When you stand, many people feel the breath not only in the belly but through the whole body, down to the soles of the feet. In the Chinese tradition this is connected with the circulation of qi known as the microcosmic orbit.

In Chanmi Qigong, a form of Buddhist Qigong, standing practice is combined with movements of the spine. The details of how to stand and breathe are something you should experience in person, because small corrections make a big difference.
So Qigong breathing and Reiki both use the breath and both know the tanden as a center, even though the energy isn't the same. Qigong knows several kinds of energy, and Reiki is spiritual life energy. Put simply, if you breathe deeply and calmly, you are already doing energy work before your hands even move. Have you ever noticed where your breath goes when you are under stress?
Experience the connection
Qigong breathing, Hara practice, the 108 Breath Meditation — in Shingon Reiki these paths flow together. Find the entry point that fits you.
Your Path in Shingon Reiki The 108 Breath Meditation