I believe that this is an example of Hirshfield and the two forces that can be found within this poem. The poem describes a moment in which she is describing this snake's behavior. But the author describes this poem as one that reflects on the abandonment that we experience within our own relationships. A figurative and literal meaning of "momentary" experiences with the people that come into our lives.
Great choice! I think this works specifically because of how the poem is framed - it’s mostly observation. The speaker’s influence is felt almost entirely through words and phrases that express barely-missed perception. That little rhetorical decision does most of the legwork here to express loss, regret, and missed opportunity.