The Hitsuzendō (筆禅道) school is a still active zen calligraphy school, originating from the art of Yamaoka Tesshū (山岡鉄舟, 1836-1888). The founder of the school was Ōmori Sōgen (大森曹玄, 1904-1994) zen master. His disciple, Terayama Tanchū (寺山旦中, 1938-2007) was head of the school until his death. He and Ōmori Sōgen together wrote a book „Zen And The Art of Calligraphy” which is a great source of information on this topic.

(Translated from here)
Translated by Judit Simon

As I previously mentioned, according to the teachings of the Hitsuzendō (筆禅道) school, ensō – the zen circle – is a fundamental element in calligraphy practice, just like mujibō (無字棒), or „zen line”, which is very much like the „zen stick”. The followers of the  Hitsuzendō school begin their practice with this single line, the brushing of which requires total concentration, the unity of body and mind, the attainment of „mindlessness” or „clear mind”. Mujibō  is the line of emptiness, like  ensō is the circle of emptiness. He who can brush either of these lines perfectly, can find in them total concentration and attention, which can be utilized later on, in other calligraphy works or sumi-e paintings. With these simple forms it will be possible to grasp the object of the soul.
Let’s take a look at two masters, Terayama sensei and Sarah Moate, brushing their ensō.

There are other teachings of this school about the deep relation between zen and artistic creation. We might often hear our calligraphy master say that all our movements should start from the hara (腹) or the tanden (丹田) so the brush can move of itself, in accord with the rhythm of the flowing life force. This idea originates from zen and ancient Chinese healing, according to which all life energy is concentrated in the centre of the body, two finger widths below the navel. This is the point from which the swordsman begins his movements, and this is the area we focus on  in zen meditation. From here the inner energy, or force „ki” (気) can flow  into the brush and through that into the ink and the paper. This life force, called bokki (墨氣 vagy 墨気), the energy of ink, shines through the calligraphies as we view them.

Finally let’s have a look at a zen calligraphy from  the book „Zen Brushwork” by Terayama sensei. It reads: „Be in the now”.