Ab initio surface-hopping dynamics simulations for the trans-penta-3,5-dieniminium cation (PSB3) are presented imposing different sets of mechanical restrictions in order to investigate the response of the molecular system to certain environmental degrees of hindrance. A general scheme for classification of photoisomerization mechanisms in conjugated chains based on the analysis of torsional angles is proposed allowing direct characterization of the different isomerization mechanisms proposed previously. On the basis of a statistical analysis of 300 trajectories a new photoisomerization mechanism-the Folding Table-was found. This mechanism and the One-Bond-Flip are almost entirely responsible for the photoisomerization process in PSB3.