As the right innominate moves anteriorly/superiorly, the sacral base moves

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At the sacroiliac joint, movement of the innominate and the sacrum occurs in opposite directions to keep the pelvis stable. When the right innominate tilts anteriorly and moves upward (anterior/superior), the sacrum responds by moving in the opposite direction—that is, it counternuttates. The base of the sacrum then moves posteriorly, and because the sacrum tilts in this interaction, the base can also appear to shift slightly upward, i.e., superiorly. This opposite motion preserves joint congruency and pelvic mechanics during movement. So, the sacral base moving posteriorly and slightly superiorly fits with the reciprocal relationship at the sacroiliac joint.