At first, the poem seems to describe an artist at work: a flash of light, a delicate brush on canvas, a quiet smile at a finished masterpiece. But the details slowly point somewhere more ordinary. The "dark instrument" with a blade, the "dark wood" and the line being made straight suggest someone shaving or planing a piece of wood, a craftsman's private, repeated task. That is an inference from the imagery, since the poem never names the tool.
The speaker watches from a distance and sounds quietly amused. The comparison to an artist is a little grand for such a small job, and the line "Man conquers nature again" feels tongue-in-cheek, as if smoothing a plank were a great victory. Even the man's "rigid spine" is mocked, hinting at someone stiff and perfectionist. The mood is calm, hushed and focused, with a thread of gentle irony running underneath it.