A Prussic Knot is an attachment device. It's a length of rope which ties to the Tether of the climber's harness and then ties to the Fixed Rope from where the Knot can be slid easily up and down the rope. It's a way to Ascend the Fixed Rope & Self-Belay your Rappel.
A "Higher Synethete" like Skippy reads that not as a Description but as Metaphor and Analogy. A Prussic Knot - like Language and all Symbols - is an Attachment Device which allows us to slide easily up and down thru meanings as we Self-Belay into Consciousness.
Uncarved block

In the days when Sussman was a novice, Minsky once came to him as he sat hacking at the PDP-6.

"What are you doing?", asked Minsky.

"I am training a randomly wired neural net to play Tic-tac-toe", Sussman replied.

"Why is the net wired randomly?", asked Minsky.

"I do not want it to have any preconceptions of how to play", Sussman said.

Minsky then shut his eyes.

"Why do you close your eyes?" Sussman asked his teacher.

"So that the room will be empty."

At that moment, Sussman was enlightened.

According to the Warlocks at Wikipedia the Koan above has its origins in the dark bowels of the Artificial Intelligence Laboratory at the Massachusetts Institute of Technololgy where Marvin Minsky and his AI Imps have been busy infusing forms of Intelligence and Consciousness into machines. Or at least thinking about how to make things think they are thinking. A form of Self-Belayed Rappeling.

“So Sussman began working on a program. Not long after, this odd-looking bald guy came over. Sussman figured the guy was going to boot him out, but instead the man sat down, asking, "Hey, what are you doing?" Sussman talked over his program with the man, Marvin Minsky. At one point in the discussion, Sussman told Minsky that he was using a certain randomizing technique in his program because he didn't want the machine to have any preconceived notions. Minsky said, "Well, it has them, it's just that you don't know what they are." It was the most profound thing Gerry Sussman had ever heard. And Minsky continued, telling him that the world is built a certain way, and the most important thing we can do with the world is avoid randomness, and figure out ways by which things can be planned. Wisdom like this has its effect on seventeen-year-old freshmen, and from then on Sussman was hooked. ”

—Steven Levy, HACKERS: Heroes of the Computer Revolution