The search for ferroelectric materials capable of producing high electric charge and power densities is important for developing a new generation of autonomous ferroelectric megawatt power supplies and ultrahigh-power-density ferroelectric energy storage devices. Here we report the results of experimental investigations of the mechanism for stress-induced depolarization of adiabatically compressing rhombohedral Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) single crystals. The important finding is that the geometric dimensions of PIN-PMN-PT crystals and the direction of adiabatic compression relative to the crystallographic orientation have significant impacts on the depolarization dynamics and the generated charge density. Based on the analysis of experimental results, we conclude that a basic depolarization mechanism is associated with stress-induced lattice distortion, which leads to the phase transformation from a polar rhombohedral R3m to a nonpolar R3c phase accounting for the complete depolarization of the crystals. We experimentally demonstrated that PIN-PMN-PT crystals are capable of releasing electric charge with a record high density, 0.47 C m-2, which is not achievable in other ferroelectric materials. The produced power density in the external load is 0.53 MW cm-3, being 2.4 times greater than that of the state-of-the-art Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 ferroelectric ceramics used in high-power systems. Therefore, PIN-PMN-PT crystals are very promising ferroelectric materials for high-power applications.