The poem begins with a still, even deathly still image. Nothing is moving. All we have is quiet, still darkness.
Berries cast dark Hooks—
Black sweet blood mouthfuls (11-13)
Nothing is innocent in this poem; even berries have a sense of foreboding to them. The speaker imagines her mouth filling up with blood as she tastes the juice of the berries. This lady's got a pretty serious dark streak.
I unpeel— Dead hands, dead stringencies. (20-21)
In this transformative moment, the speaker imagines herself stripping away the "dead" aspects of her former life—she strips away the body and the rules that are holding her down. After this, she'll be free as a bird (or… horse).
And I Am the arrow,
The dew that flies Suicidal, at one with the drive Into the red
Eye, the cauldron of morning. (26-31)
In these last few lines of the poem, the speaker imagines that she's shed her body, that she's become an arrow, that she's at "at one" with the power of her horse. This isn't a real, body-buried-six-feet-under-ground kinda death. She's imagining a more metaphorical death, one in which she transforms. To put it in happier terms: it's kinda like the death of the caterpillar who becomes a butterfly.