The study of the statistical and dynamical characteristics of extreme and very extreme events in the climate system is impaired by a strong under-sampling issue. Because extreme events are rare, answering questions about the physical mechanisms from which they arise usually depends on the investigation of just a few cases, either in observations or in models. In this paper we use a rare events algorithm to massively increase the number of extremely hot, dry and anticyclonic summers in Western Europe simulated in the state-of-the-art IPSL-CM6A-LR climate model under pre-industrial anthropogenic forcings. This allows us to reach precise climatological results on the dynamics leading to centennial hot summers. We show that these summers are characterized by both more and longer heatwaves than usual, and are associated to a combination of a local non-barotropic positive geopotential height anomaly and non-local eastward-shifted soil moisture anomalies. The atmospheric dynamics of these summers is mainly driven by a recurrence of short Rossby wave packets of synoptic scale rather than circumglobal wave patterns.