  Based on a microscopic transport model, we study the origin of nonstatistical
Intermediate Mass Fragment ($IMF$) production in semicentral heavy ion
collisions at the Fermi energies. We show that a fast, dynamical $IMF$
formation process, the {\it neck fragmentation mechanism}, can explain the
experimentally observed features: deviations from Viola systematics and
anisotropic, narrow angular distributions. It may be regarded as the
continuation of the multifragmentation mechanism towards intermediate impact
parameters. Its relation to other dynamical mechanisms, the induced fission and
the abrasion of the spectator zones, that can also contribute to mid-rapidity
$IMF$ production, is discussed. The dependence on beam energy and centrality of
the collision is carefully analysed. The competition between volume and surface
instabilities makes this mechanism very sensitive to the in-medium
nucleon-nucleon interactions, from the cross sections for hard collisions to
the compressibility and other Equation of State ($EOS$) properties.
  For charge asymmetric collisions the sensitivity of various observables to
the symmetry energy is investigated. Of particular interest appears the isospin
diffusion dynamics with no signal of isospin equilibration. However, in spite
of the short time scales and of the dynamical aspects, we notice isoscaling
features of the neck mechanism. We observe that isospin enrichement of the neck
zone as well as the isoscaling parameters are sensitive to the density
dependence of asymmetry term of EOS around and below saturation value.
