I’ve come more and more to believe in the presence and centrality of... invisible ink [in poems] — or, to use a different metaphor, to believe that there is a set of hidden clockworks beneath the surface of any poem we find ourselves moved by. This is true, paradoxically, even of poems that seem to tell everything outright. A poem may seem naked or plain, but if it moves us, there will always be something else at work, under the surface of its words. This second, undertow life is what differentiates poetry from instruction manuals, journalism, or, for that matter, a diary-type journal. Good poems always travel in more than one direction. They do not soothe us with platitude knowledge, they broaden us with complication, multiplicity, permeability to the subtle, and with unexpected perceptions, gestures of language, and comprehensions.- Jane Hirshfield, in discussion with Ami Kaye about the themes of her essays (later collected in her excellent book Hiddenness, Uncertainty, Surprise: Three Generative Energies of Poetry), over at Pirene's Fountain. You can read the whole interview here.
In addition to this larger scale dimension of hidden energies in poems, there is also a set of particular craft devices that might be described as “invisible ink.” One example is the deliberate choice to leave something out. A poem can convey an emotion or event’s presence by walking around it, revealing its shadow, alluding without naming, pressing back against it. Poems can create meaning in the same ways that mimes create walls, tables, balls, out of thin air and their own responses. This mode of communication falls into the category of what rhetoric calls periphrasis. Think of those Chinese scrolls in which the moon is a circle left uncolored. It is simply the paper, unpainted. That is an act of visual and physical periphrasis — the ink brush touches everything but the moon itself, which is, as in the physical sky, beyond any actual touch or reach.