The idea that solar wind plasma-field irregularities, i.e. plasmoids with an excess momentum density penetrate deeper into the geomagnetic field was introduced in 1976 by Lemaire and Roth at an EGS meeting. It was based on the observation that the solar wind is most of the time patchy over distances smaller than the diameter of the magnetosphere. In this early paper about "impulsive penetration", the authors did not attend to give a detailed physical description of the underlying mechanism. When Lemaire was more informed about some relevant laboratory plasma experiments carried out by Bostick, Baker and Hammel or Demidenko et al., he published in 1985 [Lemaire, J. (1985) Plasma Phys. 33, 425] a physical description of the mechanism, based on a theory first proposed by Schmidt in 1960 [Schmidt, G. (1960) Phys. Fluids 3,961]. Transient and impulsive interaction processes between the solar wind and the magnetosphere have now become an important and highly debated topic. In particular, Heikkila's argument claiming that the effects of induced electric fields are the primary cause for impulsive penetration has been shown by Owen and Cowley to be erroneous. Although the conclusions reached by Owen and Cowley [Owen C.J. and Cowley S. W. H. (1991) J.geophys. Res. 96, 5565] are correct, at least within the framework contrived by Heikkila (i.e. that of ideal MHD) [Heikkila W. J. (1982) Geophys. Res. Lett. 9,159], they do not demonstrate that real plasmoids can not penetrate impulsively onto closed geomagnetic field lines. Indeed, non-ideal MHD processes, like collective polarization effects, formation of electrostatic potential barriers, adiabatic and non-adiabatic brakings or collective diamagnetic effects, have to be taken into account in the "real world". Account of the theory of "impulsive penetration" both for weakly and strongly diamagnetic plasmoids is given, emphasizing in which respect the entry mechanism differs from ideal entry mechanisms like those proposed by Schindkr in 1979 [Schindler, G. (1979) J. geophys. Res. 84, 7257] and by Heikkila in 1982.