We experimentally demonstrate that an obstacle, when suitably placed above a bottleneck, leads to a reduction of clog formation in a silo via two different mechanisms. The first one, already suggested in previous works, is related to an alteration of the kinematic properties in the outlet proximities that prevents the stabilization of arches. The second, that is discovered when working at a quasi-static regime, appears because the obstacle induces a clear anisotropy in the contact fabric tensor. Then, both mechanisms are encompassed using a single equation in which two parameters -one related with the geometrical effects and the other to the kinematic ones- are enough to reproduce all the experimental results