The paper presents a theoretical study of unsteady wind-up of a spanwise vortex in an internal boundary layer. The authors examine the nonlinear evolution of spanwise vortex produced when local wall pressure develops a maximum or minimum, subsequent to the finite-time break-up of an interacting layer and the impact of normal pressure gradients. The evolution is controlled by an inner-outer interaction between the effects of normal pressure gradient and momentum jump across and outside the vortex, which is situated near the strong inflexion point induced in the mean flow. Although the work concentrates on a particular internal-flow context, many of the flow properties are generic and, in particular, apply to a more general case including external flows. The analysis and computations demonstrate two main distinct trends in the vortex response, depending to a large extent on a parameter gauging the relative strength of the above effects. The response is either an explosive one, provoking enhanced wind-up, growth and pressure in the vortex, or it is implosive, causing the vortex to shrink and virtually empty itself through up-winding, leaving small local pressure variation. The analysis also includes a discussion about after-effects of vortex response and about some connections with experiments and direct computations of deepening transition and stall.