The principle of Zen No-Mind is not magical. It is not that you have no knowledge and you can magically pull off martial arts techniques without training.
It means you’ve done the work, practiced your technical knowledge, so put thought into learning how to do technique correctly and then can spontaneously let that knowledge and thought operate when and how it is needed.
It means your mind is not focused on the goal but instead on doing what is right and necessary in the moment. This is exemplified in Zen in the Art of Archery, an excellent book that explains how a Westerner went to a Zen monastery in Japan and was mystified by the Zen master who told him not to try to hit the target. The target, the goal, was not what was important. The action of drawing the bow and letting it fly was what was paramount.
Too much projection, to the goal, is not real. It is imaginary. What actually exists is your action. Your action, as necessary, in the moment: The regard and poise you have, the spontaneous but refined action of technique.
By Sgt. Ethan E. Rocke, Photo ID: 2006111752721, Submitting Unit: MCB Camp Butler [Public domain
Empty mind or No-Mind means all of your attention is there without the interference of thought, without a divided or disrupted mind, and without struggle. In martial arts it happens when you have trained thoroughly, practiced and understood technique and then can effortless execute it with expert timing and precision, intensity and power. What has power looks effortless. The paradox of truth.
Featured Image: By Magyar Balázs (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons