The author here makes use of a lot of imagery, similes, and metaphores to create a vivid picture of a large feline crossing the road. Let's break it down line by line, and see what the author does.
"As I rounded the next bend, I saw a startlingly large, black feline cross the road with an unhurried, sinuous, fluid movement."- This is the first line in the passage, and already we dive into the vivid language. The author could have simply stated "As I rounded the next bend, I saw a big, black feline slowly crossing the road," but by using highly descriptive words like "starlingly large," and "unhurried, sinuous, fluid movement," we- the audience- feel the surprise at seeing such a large animal in the middle of the road, and visualize the way that it slinks in front of the car. In doing so, the author forces you to visualize something along the lines of a cougar, as opposed to, say, a particularly large house cat.
"Its thick, sinewy shoulders suggested massive strength and speed, like that of engine pistons." - To fully understand this simile, one must be able to call up a mental image of same said engine pistons. I am attaching a .gif of a radial engine, in which you can see the pistons in motion. They are parts that rotate up and down in a fluid motion. In comparing the shoulders of this feline, the author creates an image of power and agility.
"As it passed, it turned to stare at me and its great, yellow, black-slitted orbs were caught in the headlights. I noticed its pricked, tufted ears and its short, coarse, raven-black coat before it turned, raising and waving its curved snake of a tail as if making a victory salute"-
The author uses three metaphores in these two lines. First, for the animal's eyes, the authore uses "great, yellow, black-slitted orbs," in which the replacement of the word "orb" for "eye" is important. An orb, by definition, is "a celestial body, or a circle of up to 10° radius around the position of a celestial object." In this word choice, the author creates a much more supernatural feeling to the passage, which sets up quite nicely for the final lines.
Next, the author describes the feline as having a "raven-black coat." First, it gives a point of reference for just how dark the feline's coat is. Below, I have attached a picture of a raven, so that you too can connect to this visual.
Finally, the author compares the tail of the feline to a snake, creating an ominous image. Snakes (along with most other reptiles, as well as most insects) carry negative connotations- as in, they are associated with bad things. In comparing the tale of the feline to the movements of a snake, the audience is left to understand the negitive, ominous feeling that the movements of the feline invoked.
"The spectral vision dissolved into the bushes, leaving me with a thumping heart and the feeling that I had witnessed a supernatural manifestation." - In referring to the massive feline crossing as a "spectral vision that dissolved," the audience understands that the author/narrator is unsure as to whether or not the feline actually existed, or if it was simply a vision or a dream.
It is important to note that felines (particularly those that are black in coloring) are historically linked to witches and the supernatural. In the past, cats were thought to be the special familiars of witches and aid the witches in performing their magic. Today, it is still a widely held superstition that a black cat crossing your path is bad luck.