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 VS1[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.