In "My Life had stood" [. . .] Dickinson compares an action in the present tense to one in the past or present perfect:
And do I smile, such cordial light
Upon the Valley glow--
It is as a Vesuvian face
Had let its pleasure through--
And when at Night--Our good Day done—
I guard My Master's Head--
'Tis better than the Eider-Duck's
Deep Pillow--to have shared--
In the first instance, the speaker/Gun compares her smile to the aftermath of a volcanic eruption. Her smile is not like the volcano's fire or threat but like its completed act: when she smiles it is as if a volcano had erupted. The past perfect verb is more chilling than the present tense would be because it signals completion, even in the midst of a speculative ("as if') comparison; her smile has the cordiality of ash, of accomplished violence or death, not just of present fire. In the second instance, the speaker prefers guarding the master to having shared his pillow, that is, to having shared intimacy with him--primarily sexual, one would guess from the general structure of the poem. Again, the comparison contrasts action with effect rather than action with action (and when I guard . . . 'tis better than sharing ... ). As a consequence, the speaker seems ironically and almost condescendingly distant from the world of life (here, of potential life-creation or love). Shared intimacy, in her view, would bring nothing better than aggressive self-reliance does. Both uses of the perfect tense in this poem distance the speaker from humanity, perhaps as any skewed analogy would. Yet by allying herself with catastrophic power rather than sexual intimacy, she may also be indicating that the former seems more possible or safer to her; even the power of volcanoes may be known. The change in tense alerts the reader to the peculiarity and the importance of the comparisons.
From Emily Dickinson: A Poet’s Grammar. Copyright © 1987 by Harvard University Press.