Analogous to a bicycle, movement of the hub (sacrum) occurs within the wheel (innominates) by a twisting action of the rim (effect of walking). “Twisting the rim of a bicycle wheel forces the hub to tilt” [1].

The sacrum moves together with the innominates such that equal tension is continuously distributed throughout all of the sacroiliac ligaments and/or muscles. The sacrum nods anteriorly in coordination with the posterior movement of the ipsilateral ilium. At the same time, in order for tension to be maintained in the contralateral elements (ligaments, fascia, and muscles), the contralateral side of the sacrum rotates posteriorly while the ilium rotates anteriorly.

References:

1. Levin, S.M., The Sacrum in Three-Dimensional Space. Spine: State of the Art Reviews, 1995. 9(2): p. 381-88.