JD is setting up a lot of competing terms here, focusing on the center that lies within the structure and outside the structure, and the thinking of "the unthinkable itself" - making another distinction between the logical impossibility of the unorganized structure and what we're powerless to conceive during the history of the concept of structure. What he seems to be calling attention to is the way structurality projects a reassuring image of its operation: the center is what permits the play of elements, and because we confuse coherence for logical integrity, we are unable to critique the process.
That's why I decided to parse his take on structurality, because JD's distinction between structure and structurality emphasizes a distinction between instances of language and the function of structure. So in the first full paragraph on 279, when JD points out the contradictory nature of the center as that which sets the limit, structurality itself cannot be signified, or envisioned as an element.
The center becomes a contradiction: it permits and prevents play. And it is, according to JD, "contradictorily coherent." The force that binds the disparate elements of the structure with the content outside of it is "the force of desire." The easiest place to locate an idea of this desire is in Freud. Desire is what seeks fulfillment - what the center permits - and what is repressed - what the center prevents. The center permits/fulfills and prevents/represses.
"The concept of centered structure is in fact the concept of a play based on a fundamental ground, a play constituted on the basis of a fundamental immobility and a reassuring certitude, which is itself beyond the reach of play."
Play w/ a fundamental immobility = play "penned" as seen against infinite deferral. But is reassuring certitude a too-shallow understanding of our reaction to/control of language? It seems to just start the what-do-we-create vs. how-we-are-created consideration of language.
"...anxiety is a certain mode of being implicated in the game, of being caught by the game, of being as it were at stake in the game from the outset."
The double-play "of being" in each phrase makes me think I have to go back to Heidegger a little bit. Triple-play? You are being implicated/caught/at stake in the game; "being" is implicated/caught/at stake; and/or "a certain mode of being" etc.