Unlike the exclamation mark, the dash that dominates the prolific period is a horizontal stroke, on the level of this world. It both reaches out and holds at bay. Its origins in ellipsis connect it semantically to planets and cycles (rather than linear time and sequential grammatical progression), as well as to silence and the unexpressed. But to dash is also "to strike with violence so as to break into fragments; to drive impetuously forth or out, cause to rush together; to affect or qualify with an element of a different strain thrown into it; to destroy, ruin, confound, bring to nothing, frustrate, spoil; to put down on paper, throw off, or sketch, with hasty and unpremeditated vigour; to draw a pen vigorously through writing so as to erase it; [is] used as a euphemism for 'damn,' or as a kind of verbal imprecation; [or is] one of the two signals (the other being the dot) which in various combinations make up the letters of the Morse alphabet." Dickinson uses the dash to fragment language and to cause unrelated words to rush together; she qualifies conventional language with her own different strains; and she confounds editorial attempts to reduce her "dashed off " jottings to a "final" version. Not only does she draw lines through her own drafts but also through the linguistic conventions of her society, and her challenges to God are euphemistic imprecations against conventional religion. Even the allusion to the Morse alphabet is not entirely irrelevant: through her unconventional use of punctuation, particularly the dash, Dickinson creates a poetry whose interpretation becomes a process of decoding the way each fragment signals meaning.
Dickinson's transition from a dominant use of the exclamation mark to a preference for the dash accompanied her shift from ejaculatory poems, which seem outcries aimed with considerable dramatic effect at God or others, to poems where the energies exist more in the relationships between words and between the poet and her words. In this intensely prolific period, Dickinson's excessive use of dashes has been interpreted variously as the result of great stress and intense emotion, as the indication of a mental breakdown, and as a mere idiosyncratic, female habit. Though these speculations are all subject to debate, it is clear that in the early 1860s Dickinson conducted her most intense exploration of language and used punctuation to disrupt conventional linguistic relations, whether in an attempt to express inexpressible psychological states or purely to vivify language.
From "Emily Dickinson’s Volcanic Punctuation." The Emily Dickinson Journal (1993).