  In this paper we discuss the reaction mechanisms that occur in the overlap
zone for semi-peripheral heavy ion collisions at intermediate energies. In
particular we focus on the development of neck instabilities, that could
determine a possible increase of dynamical fluctuations. As observed in recent
experimental data, at beam energies just above 10 MeV/A the most relevant
expected consequence is the possibility to obtain large variances in the
projectile-like and target-like observables. With increasing beam energy we
pass to a mid-rapidity fragment production. In this way we predict a smooth
transition from a deep-inelastic to a fragmentation reaction mechanism.
