The Polanyi rules on the energy efficiency on reactivity are summarized solely from the locations of barriers on the potential energy surfaces. Here, our quantum dynamics study for the F- + CH3Cl - FCH3 + Cl- reaction shows that the two potential energy minima in the entrance channel on the potential energy surface play an essential role in energy, efficiency on reactivity. The reactivity of this reaction is dominated by the low collision energies where two distinctive reaction mechanisms involve the two minima in the entrance channel. Overall, the Cl-CH3 stretching motion and C-H-3 umbrella motion both are more efficient than the translational motion in promoting this reaction. Although this reaction has a negative energy barrier, our study shows that it is the minima in the entrance channel, together with the energy barrier relative to these minima, that determine the energy efficacy on reactivity.