MD simulation parameters are the same as in plain MD. All simulation windows start with the same AMBER restart file used in plain MD; in the restart file, the motor is at metastable form and the system has been equilibrated for plain MD. The spring constant of the window restraints were determined by trials; we found that a too large spring constant may crash the simulation, while a too small spring constant may fail to pull the reaction coordinate near the window center. The first 100 frames of each window were excluded from analysis to eliminate pulling transients.