  The main features of the gravitational dynamics of binary neutron star
systems are now well established. While the inspiral can be precisely described
in the post-Newtonian approximation, fully relativistic magneto-hydrodynamical
simulations are required to model the evolution of the merger and post-merger
phase. However, the interpretation of the numerical results can often be
non-trivial, so that toy models become a very powerful tool. Not only do they
simplify the interpretation of the post-merger dynamics, but also allow to gain
insights into the physics behind it. In this work, we construct a simple toy
model that is capable of reproducing the whole angular momentum evolution of
the post-merger remnant, from the merger to the collapse. We validate the model
against several fully general-relativistic numerical simulations employing a
genetic algorithm, and against additional constraints derived from the spectral
properties of the gravitational radiation. As a result, from the remarkably
close overlap between the model predictions and the reference simulations
within the first milliseconds after the merger, we are able to systematically
shed light on the currently open debate regarding the source of the
low-frequency peaks of the gravitational wave power spectral density.
Additionally, we also present two original relations connecting the angular
momentum of the post-merger remnant at merger and collapse to initial
properties of the system.
