Here, we propose a mechanism for the vertical force reduction in mitigated disruptions based on the stationarity of the current centroid ($Z_{curr}$) after the thermal quench (TQ), since a generally accepted explanation for the mitigation has not been established in literature previously. In short, a sufficient amount of impurities leads to a broadening of the current profile beyond the last closed flux surface, resulting in a stationary current centroid after the TQ that reduces the vertical force. Experiments of SPI mitigated VDEs in ASDEX Upgrade and JET support this theory, where we observe the stop or reversal of the centroid motion after the injection and a significant broadening of the halo current width compared to unmitigated disruptions. 2D simulations using the JOREK code were performed to confirm the theory, showing that a flattening of the current profile beyond the separatrix can effectively stop the motion of $Z_{curr}$ and reduce the vertical force related to $I_p \Delta Z_{curr}$. Overall, the simulations reproduce the $Z_{curr}$ evolution, halo width broadening, and CQ time observed in the experiments. Furthermore, predictive simulations were performed for a 15 MA ITER L-Mode scenario, which indicate that the vertical force can be reduced below 5 MN by an early injection and highlight the contribution of a nearby highly conductive wall for force reduction.