The development of extra-tropical cyclones has been investigated for a model hierarchy. The aim of the study was to get insight into the main mechanisms responsible for the rapid development. A physically intuitive approach in terms of potential vorticity has been invoked to construct so-called optimal perturbations - perturbations which amplify maximally for finite-time according to a certain metric norm. Results of idealized model experiments reveal that optimal perturbations evolve roughly according to a three-stage scenario. The mechanisms are interpreted in terms of interactions between individual potential vorticity anomalies. The novel techniques by the use of which unambiguous interpretation of initial-value experiments is possible, are easily extended and can be applied in a wide range of geophysically interesting problems exhibiting rapid development.