Using a simplified model we analyze the movement of a plasmoid injected axially into a magnetic configuration of revolution at a zero-field point of the "mono-cusp" type. One finds, in particular, that with no initial trapped field in the plasmoid, traversal, of the cusp is possible only if the injection speed of the plasmoid is greater than the Alfvén-equivalent speed. On the other hand, with an initial trapped field the cusp operates as an accelerator.