Classical trajectory studies of electron capture in ion-Rydberg atom collisions show that the total interaction can roughly be classified into two categories. In a type-1 process, the Rydberg electron is directly captured by the projectile into a stable orbit. This mechanism is very sensitive to the spatial overlap and matching velocities of the Rydberg electron and the projectile. In contrast, in a type-2 capture process the electron returns in a close passage to its own (target) nucleus after its initial atomic orbit has been perturbed by the projectile; this 'double scattering' is rather insensitive to velocity matching. The analogy between these two processes and the leading terms in a strong potential treatment of charge exchange is suggestive. Impact conditions have been identified in which the type-2 process is by far the dominant one.