We explored the mechanism of Markovnikov-selective hydrosilylation of phenylacetylene catalyzed by N-N-N Pincer-cobalt complex with density functional theory (DFT) calculations. In contrast to the previously proposed Co(I) mechanism, computational results suggest a Co(0) pathway, which is further supported by experimental studies. At the same time, our study reveals unexpected complexity in terms of the origin of regioselectivity. First, different orientations between the phenyl group in the substrate and the ligand plane lead to two possible transition states responsible for the branched product. However, the favored one varies according to ligand substitution pattern. Second, both entropy and solvation effects (rather than the conventional approach that considers electronic energies) have to be considered to explain regioselectivity, where the dominant factor also varies from case to case. Despite this complexity, computations predict a general overall ligand structure-regioselectivity relationship. In addition to increasing steric hindrance, introduction of an electron-withdrawing group to the ligands will also increase regioselectivity, which unveils a new dimension of ligand design.