The dynamic role of the conical intersection "seam" coordinate has been first revealed in the H fragmentation reaction of ortho(o)-cresol conformers. One of the (3N - 8) dimensional seam coordinates of the S-1(pi pi*)/S-2(pi sigma*) conical intersection has been identified as the CH3 torsional potential function. The tunneling dynamics of the reactive flux is dictated by its nuclear layout with respect to the CH3 torsional angle, as the multidimensional tunneling barrier is dynamically shaped along the conical intersection seam. The effective tunneling-barrier weight-averaged over the quantum-mechanical probability along the CH3 torsional angle perfectly explains the experimental finding: the sharp variation of the tunneling rate ((700-400) ps(-1)) with the CH3 torsional mode excitations within the narrow (0-100 cm(-1)) energetic window. The much longer S-1 lifetime of cis compared to trans is ascribed to the higher-lying S-1/S-2 conical intersection of the former. With the use of distinct lifetimes, vibronic bands of each conformer could be completely separated.