To elaborate on Zeiss Ikon's answerZeiss Ikon's answer, I have found occurrences of negotiating, taking and walking the Logrus; all of these terms also apply to the Pattern, so differences between the two probably lie elsewhere.
“At some point, the Pattern discovered—or perhaps possessed all along—the ability to create ‘ghosts,’ short-lived simulacra of those who had negotiated it.” [Prince of Chaos, chapter 4]
Prince of Chaos, chapter 4
“They have a sort of equivalent called the Logrus. It’s a kind of chaotic maze. Keeps shifting about. Very dangerous. Unbalances you mentally, too, for a time. No fun.” [Trumps of Doom, chapter 6]
Trumps of Doom, chapter 6
“No. The Logrus is—” he hesitated, as if searching for the words to describe the indescribable. “It is a key to finding your way amongst all the Shadow worlds. It is like a maze. By traversing its length, from start to finish, someone born of Chaos may have the Logrus imprinted on his mind forever.” [The Dawn of Amber, chapter 8]
The Dawn of Amber, chapter 8
Although its most common form is definitely to be the tentacular shape it takes when it is used to retrieve objects from Shadow, or when summoned:.
I closed my eyes and visualized an image of the Logrus shifting, ever shifting. I framed my desire and two of the swimming lines within the eidolon increased in brightness and thickness. I moved my arms, slowly, imitating their undulations, their jerkings. Finally, the lines and my arms seemed to be one, and I opened my hands and extended the lines outward, outward through Shadow. [Trumps of Doom, chapter 6]
Trumps of Doom, chapter 6
Then, as she neared the stairway, the Sign of the Logrus appeared before her, larger than any I’d ever summoned, filling the corridor from wall to wall, roiling, sprawling, fire-shot, tentacular, a reddish haze of menace drifting about it. [Knight of Shadows, chapter 10]
Knight of Shadows, chapter 10
“I can offer you cosmic greatness.”
“You
“You already did. I turned you down then, too. Remember?”
“I
“I remember. And I will remember.” A jagged tentacle of the constantly shifting figure moved toward one of the circles of light. There was a blinding rush of flame when they met. [Prince of Chaos, chapter 12]
Prince of Chaos, chapter 12
My personal opinion about negotiating the Logrus is that it involves traversing the maze made of these threads—waving threads — exploring the paths to become one with the Logrus' power. Just like the Pattern, but the threads would be made of power, without a physical representation.
“And once in the Courts, you must survive the Logrus. Not all of us do, you know. My brother died on his first attempt. It destroyed him, mind and body. It is not so simple a matter after all.” [The Dawn of Amber, chapter 8]
The Dawn of Amber, chapter 8
“Not really. The Logrus actually alters itself somewhat, constantly. Still, it’s more angular, whereas this is mostly curves and bends.” [Blood of Amber, chapter 9]
Blood of Amber, chapter 9