God's lioness, How one we grow, Pivot of heels and knees!—The furrow
Splits and passes, sister to The brown arc Of the neck I cannot catch,
Were you hoping for a bit more solid ground in these next stanzas?
Sorry kiddos, it looks like you're out of luck. Ariel has taken off at an uncontrollable gallop, and we're losing our grip on what's happening right along with the speaker. By not explaining her every move, by not filling in all of the details, the speaker brings us right atop Ariel with her for a disorienting ride.
In other words: feeling like you've lost your grip on the poem is kinda the whole point of the poem.
So in this stanza, we're actually introduced to Ariel—the speaker's horse. The speaker calls her "God's lioness," perhaps to make her seem fierce, even otherworldly. (Let's face it: lionesses are way fiercer than horses.)
Then, in the following lines, we are able to put together Ariel's appearance in a piecemeal way. We see the "pivot"—the movement—of the horse's "heels and knees." We see "the brown arc / Of the neck," which is "sister," or somehow alike to, the "furrow" or trail in the ground below. We've got a slightly clearer picture of Ariel now.
We see these images as flashes, as the speaker does. Remember, we're galloping at a pretty fast pace. And it's hard to hold on; the speaker tells us that she "cannot catch" Ariel's neck. She has literally lost her grip on the horse.
A brief note on form: did you notice all of Plath's enjambments? The way the lines break weirdly, against the natural grain of the sentence? All of these enjambments give the poem a rushed feel, as if the speaker has no time to compose her lines into neat little contained phrases: "The furrow / Splits and passes, sister to / The brown arc / Of the neck I cannot catch." The form of the poem is really matching up with the content in these lines. The rushed enjambments make us feel like we're being taken on a wild, rushed ride too. (And in a way, we are!) (Check out "Form and Meter" for more on this technique.)
Before we move on, let's once again note how dense the poem sounds—just check out the thickness of the repeated hard C consonance in "arc," "neck," and "catch," for example. (Check out "Sound Check" for more on this.) Each image that we flash on has a certain sonic depth in addition to a visual depth. This is part of the reason that reading this poem makes us feel like we're actually sitting astride the galloping Ariel, holding on for our lives.