"As a general term, sinking mind denotes any dimming of awareness. at its best it is sort of a mental vacuum in which there is no thought, no observation of the breath, no awareness of anything. It is a gap, a formless mental gray area rather like a dreamless sleep. Sinking mind is a void. Avoid it."
As a person who meditates regularly, I wholly disagree with the 'sinking mind' concept and avoidance of it. The place where there is no thought or observation of breath is EXACTLY what you strive to achieve so you can finally allow the vibration to rise and allow the Inner Being a voice which is only heard in such a void whilst meditating. The place where you are there in a meditative state, awake yet disconnected from sensation or thought is the allowance - the grace from the monkey mind - to feel yourself gaining perspective that is above the nonsense that is regular mental chatter.