The PM3(tm) semiempirical method was used to show the effect, of a second ethylene molecule in the backside position on the frontside ethylene insertion in the Cp2Zr-Pr-n(+) gamma-agostic resting state. The same calculations without a companion second ethylene molecule were performed to compare geometrical parameters, energies, and electrostatic charges. The results obtained show that the geometrical parameters for both cases were identical, but differences in the electrostatic charges were observed. The energy profile presented two barriers, the first corresponding to the alkyl-chain rotation along the Zr-C alpha bond and the second relating to the insertion process itself. The presence of a companion second ethylene molecule affected the energetic profile by lowering the energy barrier of the first stage with respect to the process without the companion second ethylene molecule. These results provide some theoretical support to the well-known trigger effect.