This excellent poem by Emily Dickinson uses the extended metaphor of a funeral to describe the experience of the speaker which we can perhaps compare to a complete mental breakdown. At each stage, the separate sections of a funeral are related to the mental state of the speaker until the shocking finish of the poem. Thus in the first stanza the "Mourners" treading to and fro and the "Service, like a Drum" is related to a repeated beating sound going on in the speaker's brain that makes the speaker's mind "go numb." The box that is lifted "creaks" across the soul of the speaker. As she imagines the coffin to be placed on top of a plank, that plank suddenly breaks, and the coffin is left to plummet downwards towards an unspecified state:

And then a Plank in Reason, broke,
And I dropped down, and down--
And hit a World, at every plunge,
And Finished knowing--then--

The dash seems to contradict the words that come before it, because, instead of "finishing knowing," the poem is left unfinished, the dash suggesting that there is something yet to come, which might reflect the ambiguous ending of the poem. However, at every stage we can see that the funeral process is related to the descent of the speaker into some sort of mental oblivion.