“A narrow Fellow in the Grass” reexamines two amazing snake-sightings that would otherwise seem very normal to a person without a poet’s eye for imagery. The first amazing sighting of the poem is when the speaker almost steps on a snake, and the second is mistaking a different snake for a leather whip. In both cases, most folks would keep on walking, but our speaker relates these as extraordinary events. Why? Well, because in a way they are extraordinary. What happens to the mind in the moment that assumption and expectation are undone? Is there a clear line between one world in nature, and another in the human realm? These are super-big-picture questions (like, billboard-sized), and they don't have any real, immediate answers. Our speaker doesn't seem to mind, though. He's content to live in the sensation that such questions and experiences produce. And in this poem, that sensation is best described as awe and amazement.
The strange word order in lines 3 and 4 creates this jerky feeling through incomplete thoughts and a seemingly misplaced question mark. The twisted syntax here, and elsewhere in the poem, mimics the way seeing a snake would make you jump. Look out!
For the speaker, it's impossible to be amazed without being deeply disturbed and afraid at the same time. That explains the poem's final, shiver-ific stanza.