MuShin” is comprised of two kanji characters - 無 (Mu): “Nothingness”, and 心 (Shin): "Heart, mind or self-consciousness".
The concept of MuShin is prevalent in many Japanese arts and may be said to have its origins traced back to Zen philosophy.
How does one even do it in the practice of Iaido? At first glance, it seems contradictory. To improve, practice requires mindfulness and dedicated practice towards perfection. Yet MuShin sounds as though to empty oneself of thoughts...?
The funny thing is, all requires dedicated practice. A solid familiarity with footwork and kamae (body posture) is a firm must to begin with. The next ingredient is fluidity in kata (muscle memory and cognitive reflexes). With these two in place, the more one practices mindfully, the more one comes closer to the state of "MuShin".
A similar analogy would be how one learn to swim, to ride a bicycle, to juggle, etc. The more we practice, the more we can be conscious of our movements. But we are able to observe these nuances calmly, make adjustments accordingly and ultimately not be perturbed by it. This is a goal to reach in practice, lofty as it might seem. In a certain extent, mindful mediation to clear and settle the mind can help towards this endeavor.
A similar way of contemplating this is the common saying of appreciating the entirety of the forest, not being fixated on a single tree and thus missing out on the forest.
It helps to appreciate this nugget of wisdom, especially when we are stuck ironing kinks on a particular waza or have hit a plateau. It prompts us to consider what might we have been overlooking.