This struck me from an interview with Gluck. (Apparently she rarely gives interviews) in this month's Poets and Writers magazine:
"For me it's tone--the way the mind moves as it performs its acts of meditation. That's what you're following. It guides you but also mystifies you because you can't turn it into conscious principles or say precisely what its attributes are. The minute you turn tone into conscious principle it goes dead. It has to remain mysterious to you. You have to be surprised by what it is capable of unveiling."
That's what I love about writing poetry. The work itself unveiling to you, the work itself letting you know what it needs to say.