Sûsokukan 数息観 is the breath-counting meditation of Shingon Buddhism. You sit, you breathe, and you count your breaths up to 108. There's no visualization, no mantra and no ritual involved. That sounds almost too simple, and yet anyone who has tried to stay with 108 breaths really attentively knows that the mind has other plans. It jumps, it comments, it starts telling stories. This is exactly why Sûsokukan is used as a foundation for the deeper meditations of Shingon.

What is Sûsokukan? 数息観
The character kan 観 is the interesting one here. It's the same kan you find in Gachirinkan, the moon disc contemplation, and in Ajikan, the contemplation of the Siddham syllable A (Bonji in Japanese). In Shingon, kan stands for a way of looking that stays with one object, and in Sûsokukan that object is simply the breath. Breath-counting isn't an invention of a modern mindfulness movement, by the way. It reaches far back into the Buddhist meditation traditions of India and China and came to Japan along with them.
The principle is easy to describe. You count each breath, and when you notice that your mind has wandered off, you start again at one. Most people get lost somewhere around the third or fourth breath. That moment of noticing and coming back is the actual practice. It isn't a failure, it's what the exercise is for.
Posture and hands 坐
Sûsokukan is practiced in an upright, stable sitting posture, whether cross-legged, in seiza or on a chair. The hands usually rest in the lap in the meditation mudra Hokkai jōin 法界定印, the "meditation seal of the Dharma realm." Strictly speaking, jō 定 is the Japanese term for samādhi, the gathered state of meditation. So the name of the gesture already tells you what it's about.
I won't go into the details of breathing and counting here. These belong in practice with guidance, because small things like the length of the exhale or the rhythm of the count make a difference that is hard to get across in writing.

Why 108? 百八
108 is one of the most important numbers in Buddhism. If you spend New Year's Eve at a Japanese temple, you'll hear the temple bell struck 108 times. This is called joya no kane 除夜の鐘, and each strike stands for one of the 108 bonnō 煩悩, the passions and delusions that cloud the mind. A full Buddhist rosary (juzu 数珠) also has 108 beads. Yes, you read that right: the ju 数 in juzu is the same character as the sū in Sûsokukan, "to count."
How exactly one arrives at 108 is explained in different ways within Buddhism. One common calculation multiplies the six senses (seeing, hearing, smelling, tasting, touching and thinking) by different kinds of feeling and by the three times of past, present and future. There are several such calculations, and none of them is the only correct one. What they all have in common is the idea that 108 stands for the whole range of what binds the mind. Counting 108 breaths means going through that whole range once, quietly and without a fight.
The entry point, and what comes after 門
In my own practice, Sûsokukan is one of the foundation meditations of Kuji Kiri 1, together with the moon disc contemplation (Gachirinkan) and Ajikan. In Kuji Kiri 9 it comes back again, which closes the circle. The reason is simple: mantra, mudra and Siddham contemplation all need a mind that can stay with one thing, and Sûsokukan trains exactly that.
The inner resistances that show up during practice, like restlessness, boredom or sudden tiredness, aren't disturbances that you have to get rid of first. In the Buddhist view they are part of the bonnō themselves, and every breath you count anyway is a small act of clearing.
Anyone can start with breath-counting. The deeper layers that build on it, such as the contemplations with Siddham and mantra, are opened through initiation and practiced in the community and at the live events. Have you ever tried to count 108 breaths without losing track?
Your entry into Shingon meditation
From breath practice to Siddham contemplation: how Shingon meditation builds layer by layer.
Your Path in Shingon Reiki The Moon Disc Meditation