In this excerpt, the spiritual director uses the image of being wood to a carpenter. The idea is that the practitioner of Centering Prayer is passive. The goal is not to "do something," but to let something be done in you. To let that mysterious grace move.
I come back to this image often.
"...become increasingly faithful to this work until it becomes your whole life.
To put it more simply, let that mysterious grace move in your spirit as it will and follow it wherever it leads you. Let it be the active doer and you the passive receiver. Do not meddle with it (as if you could possibly improve on grace), but let it be for fear you spoil it entirely. Your part is to be as wood to a carpenter or a home to a dweller. Remain blind during this time cutting away all desire to know, for knowledge is a hindrance here. Be content to feel this mysterious grace sweetly awaken in the depths of your spirit. Forget everything but God and fix on him your naked desire, your longing stripped of all self-interest."
– The Cloud of Unknowing, Chapter 34