An intuitive explanation of this process in terms of motions on the ground and excited state potential energy surfaces (PES) is provided in Fig 2 . The motor starts in stable form (A) and is excited by light to a Franck-Condon (FC) point. It then rotates down the gradient of the excited state PES. During this critical step, it "flies over" a large ground state energy barrier that would be very difficult to pass on the ground state PES. When it is near the conical intersection between the excited state and ground state surfaces, the molecule hops back to its ground state and continues down the energy gradient to the metastable form (B). Because there is now a high ground state energy barrier blocking a reverse motion back to A, the motor remains in B until thermal fluctuations kick it over the lower THI barrier to stable form (C). From C, the same process repeats, moving the motor through metastable form (D) and back to stable form (A).