This form of meditation is a little different from both Candle and Mirror meditation in that your mind is engaged more actively, yet Mushin Meditation does not require the same level of mental concentration as Waterfall Meditation. In this meditation you will consciously use your mind to let go of any and all thoughts as they arise rather than passively waiting for your mind to clear through prolonged practice. Or, to put it another way, you will use your thoughts to clear your mind of thoughts.
The aim here is, as always, to quieten your left brain hemisphere in order to gain and prolong access to your right hemisphere. As you consciously let go of conceptual (left hemisphere) thought you will find yourself becoming more and more aware of the different, more intuitive and non-conceptual 'thoughts' originating in your right hemisphere. This will lead you to a direct and ever-deepening experience of mushin or no mind, or, more accurately still, no conscious mind. This is not the same as satori (realization or 'enlightenment') but is a generally accepted prior experience and one which will benefit both your life in general and your practice of martial arts in particular. In short, Mushin Meditation will lead you to a state of no thought.
Mushin
'The Sword and Zen are One' - Tsuji Gettan Sukemochi
What follows is simple to describe and understand but can provide a lifelong quest to master. As different thoughts arise in your mind and come to your attention, rather than allow them to take root and begin to absorb you, concentrate your mind and consciously let those thoughts go. This should not be a strenuous exercise, but a gentle, albeit conscious, one. As paradoxical as it is, you want to be using your thoughts to remove thoughts. Through time you will become more sensitive to the moments between thoughts and such times will occur more frequently and for a longer duration. This is a state of mushin and is the goal you are reaching for.
Continue in this manner for 5-10 minutes, twice a day if possible. In the early stages you will most likely find your mind continually coughing up distractions. Thoughts will seem to arise from nowhere. This is okay. Just let them go. You will enjoy brief moments of thinking of nothing before your mind suddenly realizes it has cleared and will once again demand to be fixated on a new thought. As that new thought comes, let it go. With the practice of Mushin Meditation over time you will find the moments of no thought become more frequent and last longer. In those moments of mental stillness just stay relaxed and caught up in that moment for as long as you can. In such moments expect to lose awareness of the passing of time and of yourself as a thinking person.
Bodhidharma, the founder of Ch'an / Zen Buddhism
When you are ready to restore full conscious thought, gently begin moving your body again. Give yourself a light massage to help the blood flow. Avoid suddenly throwing yourself into a frantically paced lifestyle again but rather enjoy the lingering effects of the peace of mind you have enjoyed. Sip a cool glass of apple juice or listen to some soothing music. Take your time to savor the connection you had made with yourself.