The working mechanism of a geometrically overcrowded, chiral stilbene light-driven molecular rotary motor [(2R,2R)-2,2',7,7'-tetramethyl-1,1'-bis(indanylidene), 3] has been investigated by a potential energy surface (PES) study. The reaction paths of the two photoinitiated cis-trans (or E/Z) isomerization processes, namely, (P,P)-stable-cis -> 4(M,M)-unstable-trans-3 and (P,P)-stable-trans ->(M,M)-unstable-cis-3, have been explored at the CASPT2//CASSCF level of theory. The minimal energy reaction paths (MEPs) of these two processes are nearly parallel on the PESs, separated by a ridge of high inversion barrier. The MEPs have a remarkably steep slope, which drives C=C bond rotation unidirectionally. The asymmetric bias on the excited-state MEPs is caused by the substituents on the "fiord" region as well as by the phenyl moieties. The overall photoisomerization reaction can be described as a three-state (1B -> 2A -> 1A) multimode mechanism: The molecule excited to the 1B state first crosses one of the sloped 1B/2A seams, and then follows two cooperative torsional reaction modes to cross preferentially one of the two 2A/1A conical intersections to reach the isomerized ground-state product.