Shodo is a Japanese word and can be translated literally as ‘the way of writing.’ The word refers to a form of calligraphy (‘beautiful writing’) that is used for writing Japanese characters (in particular kanji) with a brush and ink on rice paper. 

Like other Japanese Arts that end in -do (‘way,’ ‘path’) – such as Aikido, Iaido, Karatedo, Judo, Budo, Kado (‘the way of flower arrangement’ – also called Ikebana), or Sado (‘the way of tea,’ i.e. ‘tea ceremony’ – frequently also referred to as Chanoyu, ‘hot-water tea,’ Chaji, or

Aikido - Calligraphy
by Kataoka Sensei

The basis – or “key” – for this connection is Ki, a complex Japanese term that comprises the English
ideas for ‘spirit,’ ‘breath,’ ‘energy’ (in particular ‘universal energy’ or ‘energy flow’), and ‘life force.’ It is comparable to the Sanskrit Prana (‘breath,’ as in Pranayama, the basic Yoga breathing method), the Greek πνεῦμα(‘air,’ ‘breath,’ ‘spirit’) and the Latin Spiritus (which originally meant ‘breathing’ as well).

All Japanese traditional art forms or do […] are based upon a common breathing method,

 

Ki is the enigmatic and
dynamic force behind
beautifully powerful
calligraphy and painting.

Similarly, Kyle Weaner points out:

Technical mastery with the brush is only the first step. For the image to come alive, the calligrapher must imbue […] [it with] something of his soul, his creative energy.

 

In order for this to happen, the calligrapher has to get into a certain state of mind:

[…] true creativity is not the product of consciousness but rather the phenomenon of life itself.
True creation […] must arise from mu-shin, the state of ‘no mind,’ in which thought, emotions, and expectations do not matter.

Truly skillful Zen calligraphy is not the product of intense ‘practice;’ rather, it is best achieved as the product of the ‘no-mind’ state, a high level of spirituality, and a heart free of disturbances

In this state, the life energy – or Ki – can flow freely through the artist’s body and manifest itself in the movement of the brush – and thus in the trace the ink that is on the brush leaves on the paper:

Shodo allows the dynamic movement of the artist's
spirit to become observable in the form of rich
black ink.
In shodo, you can sense both the rhythm of music
as well as the smooth, elegant, and balanced
construction of architecture. Many practitioners
feel that the ‘visible rhythm’ of Japanese calligraphy embodies a ‘picture of the mind’ – and calligraphers recognize that it discloses our spiritual state.
This recognition is summed up by the traditional
Japanese saying: Kokoro tadashikereba sunawachi
fude tadash i—‘If your mind is correct, the brush
will be correct.’

What helps the mind – and the brush – to be “correct” is awareness because it allows the cosmic energetic vibrations to enter the calligrapher and to flow through him or her into the brush and onto the paper.
As Peter Vernon  Quenter points out:

Awareness of the present moment flows through mind
and body, brush and ink, onto the delicate and fragile recording surface of natural paper.

Shodo Artist and Teacher Solana Yuko Halada describes the same phenomenon from a slightly different angle. She says,

When I work within a ‘No Mind’ zone, which is a
concept of Zen Buddhism, I am enabled to transmit
God’s energy through my being into my brush, and
am guided to create shodo that incorporates the messages
flowing out from divine energy. When people view my calligraphy they receive, consciously or unconsciously, these messages and sense the writing’s inner vibrations:  this can lead to a positive shift in their awareness and insights.

Kataoka Sensei and her Shodo Work –  April 2008

Nagaya Roshi sums up this complex process when he remarks:

The trace of the ink is not merely beautiful script.
It is something that emerges from the fundamental
source of being.

When you trace the script of a character you must
Be of that original essence.