We study the evolution of a magnetic arcade that is anchored to an accretion disk and is sheared by the di †erential rotation of a Keplerian disk. By including an extremely low external plasma pressure at large distances, we obtain a sequence of axisymmetric magnetostatic equilibria and show that there is a fundamental di †erence between Ðeld lines that are a †ected by the plasma pressure and those that are not (i.e., force free). Force-free Ðelds, while being twisted by the di †erential rotation of the disk, expand outward at an angle of D60¡ away from the rotation axis, consistent with the previous studies. These force-free Ðeld lines, however, are enclosed by the outer Ðeld lines, which originate from small disk radii and come back to the disk at large radii. These outer Ðelds experience most of the twist, and they are also a †ected most by the external plasma pressure. At large cylindrical radial distances, magnetic pressure and plasma pressure are comparable so that any further radial expansion of magnetic Ðelds is prevented or slowed down greatly by this pressure. This hindrance to cylindrical radial expansion causes most of the added twist to be distributed on the ascending portion of the Ðeld lines, close to the rotation axis. Since these Ðeld lines are twisted most, the increasing ratio of the toroidal component to the B Õ poloidal component eventually results in the collimation of magnetic energy and Ñux around the B R,z rotation axis. We discuss the difficulty with adding a large number of twists within the limitations of the magnetostatic approximation.