The speaker picks at a single word, "instant," and refuses to accept it. The poem opens with a simple question: how long is an instant, a second, five seconds? But it quickly becomes clear that the speaker has something heavier in mind. They imagine a moment in which pain, horror, realisation and sorrow all "come crushing together," and suspect that this moment is far from tiny.
The target seems to be a common reassurance that a death, or a moment of suffocation and struggle, is "instant" and painless. The speaker pushes back: if you didn't know how that instant felt, how can anyone say it hurt nothing? The repeated "no pain, no pain" sounds more like someone trying to convince themselves than a comfort.
The mood is restless, bitter and unsettled. Short, broken lines make the speaker sound stubborn and a little raw, and the closing claim that the idea is "contradictory" feels like a refusal to be soothed by an easy phrase.