The Ovonic Threshold Switching (OTS) phenomenon, a unique discontinuity of conductivity upon electric-field application, has been observed in many chalcogenide glasses, some of which being presently used as selector elements in latest ultimate phase-change memory devices. In this work, Ab Initio Molecular Dynamics is used to simulate the structure of two prototypical glasses that have been shown to exhibit significantly different OTS properties and switching performance in OTS devices. The first glass, Ge30Se70 has a typical structure of connected Ge tetrahedra, whereas in the second Ge30Se70-based glass that contains antimony and nitrogen, the structure around Ge atoms is quite more complex. By the simulation of the excitation of electrons in the conduction band, slight modifications of the local order are shown to be sufficient to delocalize electronic states. The electron delocalization involving both Ge, Se (as well as Sb atoms in case of Sb-containing glass) ensures the percolation of conductive paths for electrons giving therefore to the excited material a metallic behavior. These conductive channels result from the local formation of 'metavalent' bonds [1, 2] in the amorphous structure as characterized geometrically and with associated Born effective charges.