Interacting galaxies and mergers are key events in galaxy evolution, and are often associated with boosts of the star formation activity, in particular in the form of massive clusters. Observations still struggle to identify the main driver(s) of these starbursts and it is still not clear whether they are triggered by an increased mass of dense gas, and/or an increased star formation efficiencies. Knowing which process dominates in which physical conditions is key to extrapolate our understanding of star formation to the early Universe. In this talk, I will use simulations of interacting galaxies, at low and high redshift, to pin down the underlying physics of starbursts. I will show that complex combinations of physical processes are playing different roles along the evolution of a merger, but also across a given galaxy, and account for a diversity of efficiencies. I will demonstrate that, despite having comparable outcomes in the star formation rates, these different physical mechanisms leave different signatures, for instance in the CO emission and the formation of massive clusters.