What does this equation mean?

a shift transverse to both the spin and the relative velocity [ 13 ] . This is an exact identity of special relativity, not an approximation valid only for small v ; it holds all the way to v $\to$ c , at which point |$\Delta$$\mathbf x$| $\to$ |$\mathbf S$|/(Mc) = $\rho_M$ exactly, since |$\mathbf S$ $\times$ $\mathbf v$| $\le$ |$\mathbf S$|\,v saturates when the spin is perpendicular to the boost. No boosted observer, however extreme, can push a body’s own centroid further from its rest-frame value than $\rho_M$ . As u ranges over every inertial observer, $X^\mu(u,\Sigma)$ traces out a disk of exactly that radius, orthogonal to $\mathbf S$ — the world-tube first identified by Møller and reproduced in modern language by Costa and Natário’s review of spin supplementary conditions [ 13 ] .