The post-stishovite transition has been studied using high-pressure spontaneous strains, optic modes, and elastic moduli based on the Landau modeling, but its atomistic transformation mechanism remains unclear. Here we have conducted synchrotron single-crystal X-ray diffraction measurements on stishovite crystals up to 75.3 GPa in a diamond-anvil cell. Analysis of data reveals atomic positions, bond lengths, bond angles, and variations of SiO₆ octahedra across the transition at high pressure. Our results show that the oxygen coordinates split at approximately 51.4 GPa where the apical and equatorial Si-O bond lengths cross over, the SiO₆ octahedral distortion vanishes, and the SiO₆ octahedra start to rotate about the c axis. These results are used to correlate with elastic moduli and Landau parameters in the symmetry-breaking strain e₁ - e₂ and order parameter Q to reveal the atomistic origin of the ferroelastic transition. When the bond lengths of two Si-O bonds are equal, the elastic modulus C₁₁ converges with the C₁₂ and the shear wave V_S1[110] propagating along [10] and polarizing along [110] vanishes. The e₁ - e₂ and Q are proportional to the SiO₆ rotation angle. Our results on the pseudo-proper type transition are also compared with that for the proper-type in albite and improper-type in CaSiO₃ perovskite to shed new light on transition mechanisms in other types of the ferroelastic transitions. The symmetry-breaking strain in all these types of transitions arises as the primary effect from the structural angle, such as SiO₆ rotation or lattice constant angle, in the low-symmetry ferroelastic phase.