Hishiryo is a state of mind beyond thinking and non-thinking. During Zazen, it is the normal condition of the consciousness.
Consciousness during zen meditation is not the same as in daily life, it does not behave like the intellect. During zazen, we have thoughts that appears and disappears naturally, this is perfectly normal. If we let those thoughts come and go freely, without giving form to those thought, without wanting to chase them, the intellect becomes peaceful by itself and hishiryo consciousness appears, beyond thinking and non-thinking.
When you are thinking constantly, you are not in the mind's normal condition. Your imagination and personal desires are expressing themselves to your mind, taking you away from your mind's simple, peaceful, original condition.
The more you think, the more afraid you become, and the more anxious you grow. After too long, complications and even madness can arise from this.
When you think too much, conflicts and battles take hold of your mind, preventing you from ever finding true peace. This prevents you from harmonizing with reality, and with the Universe as a whole.
When you stop this internal activity, you return to the normal, simple, and peaceful condition of the mind.
We can observe the simplicity of Hishiryo in animals. Cats are, for example, very simple creatures. They are easy to understand and are not complicated. A cat does as it pleases, and it is completely connected with the present, never overthinking.
"The more you think, the more afraid you become, and the more anxious you grow (...)."
Animals live true Zen, as they are totally connected with the present moment, and by extension, the Cosmos. Thus, we should look to them as examples in our journey to become one with the Universe.
Hishiryo is the absolute egolessness consciousness, in full unity with the whole Universe. and like Zen, it can cannot be explained but it can be experienced during zazen, naturally and unconsciously.