During a major merger event, a substantial fraction of the galactic tides can be totally compressive. A molecular cloud sitting in such a region could form a young cluster and 'cocoon' its formation. However, when the tide switches to the more likely extensive mode, the proto-cluster could suffer important mass-loss or even early dissolution. This will strongly affect the mass function. Furthermore, the compressive tides could keep the gas bound, even during the feedback of the first stars. Thus, compressive regions are likely to be the site of embedded star clusters formation. Using N-body simulations of the Antennae galaxies, we derived the characteristics in space and time of these compressive modes. We compared these with recent observational data of young clusters and found a very good agreement.