Because the gravitational line falls anterior to the transverse axis of sacral rotation (the axial sacroiliac joint) and posterior to the transverse axis of innominate rotation (the acetabulum), gravity causes a counter-rotation of these two bones [1] [2]p3-5 [3] [4]p153-158. In this movement, known as nutation (from the Latin word nutare, “to nod”), the sacrum appears to move anteriorly and inferiorly, as the ilium appears to move posteriorly and inferiorly. In the alternating motion, counternutation, the sacrum appears to move superiorly and posteriorly as the ilium appears to move superiorly and anteriorly [1, 3, 5-10] [2]p3-5 [11]p64-67 [4]p153-158.

During nutation, when force is transmitted superiorly through the legs or inferiorly through the spine, a spring like action takes place. The sacrum rotates anteriorly and inferiorly, while the ilium rotates in the opposite direction, posteriorly and inferiorly. Below the sacrum, the feet pronate, the fibulae move inferiorly [12], and the knees twist as the tibia goes farther into internal rotation than the femur. At the sacroiliac joint, this movement winds the interosseous ligament, drawing the sacrum and ilium closer [7] [11]p56 [13]p55, but not together [10]. Above the sacrum, the lumbar curve increases like a compressing spring [14]p54&60 as the sacroiliac joint is forced into nutation.

Once the energy is absorbed and the end of range of motion is reached, the body reacts by going into counternutation, and all the above actions are reversed as the spring rebounds. Through this mechanism, the sacroiliac ligaments can be seen to be not only the center of motion in the body, where the upper body meets the lower body but, also, the central spring in shock absorption [15, 16] [17]p438.

From this, we can see the susceptibility of the sacroiliac joint to the nutation lesion. Basically, any force directed axially from the trunk downward will force the sacral base anteriorly and inferiorly, away from the ilia. Likewise, any force directed upward through the legs will force the ilia posteriorly and inferiorly, away from the sacral base.

When the force is greater than the ligaments can withstand, the ligaments will tear, resulting in the sacroiliac nutation lesion. The central spring in the shock absorber system becomes damaged and the normal mechanism of weight bearing becomes compromised (see The Nutation Lesion).

References:

1. Weisl, H., The ligaments of the sacroiliac joint examined with particular reference to their function. Acta Anatomica (Basel), 1954a. 20(3): p. 201-13.
2. Jungmann, M. and C.W. McClure, Backaches, Postural Decline, Aging, and Gravity-Strain. 1963, New York, NY: The Institute for Gravitational Strain Pathology, Inc.
3. Dontigny, R.L., Dysfunction of the sacroiliac joint and its treatment*. The Journal of Orthopaedic and Sports Physical Therapy, 1979. 1(1): p. 23-35.
4. Porterfield, J. and C. DeRosa, Mechanical Low Back Pain: Perspectives in Functional Anatomy. 2nd ed. 1998: W B. Saunders Company.
5. Weisl, H., The articular surfaces of the sacro-iliac joint and their relation to the movements of the sacrum. Acta Anatomica (Basel), 1954b. 22(1): p. 1-14.
6. Weisl, H., The movements of the sacroiliac joint. Acta Anatomica (Basel), 1955. 23(1): p. 80-91.
7. Solonen, K.A., The sacroiliac joint in the light of anatomical, roentgenological and clinical studies. Acta Orthopaedica Scandinavica. Supplementum, 1957. 27(Suppl 27): p. 1-127.
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11. Kapandji, I.A., The Physiology of the Joints. Vol. 3. 1977: Churchill Livingstone.
12. Weinert, C.R., Jr., J.H. McMaster, and R.J. Ferguson, Dynamic function of the human fibula. The American Journal of Anatomy, 1973. 138(2): p. 145-9.
13. Vleeming, A., et al., The role of the sacroiliac joints in coupling between spine, pelvis, legs and arms., in Movement, Stability, and Low Back Pain, A. Vleeming, et al., Editors. 1997, Churchill Livingstone. p. 53-71.
14. Vleeming, A., et al., eds. Movement, Stability, & Low Back Pain. 1997, Churchill Livingstone.
15. Wilder, D.G., M.H. Pope, and J.W. Frymoyer, The functional topography of the sacroiliac joint. Spine, 1980. 5(6): p. 575-9.
16. Grieve, E.F., Mechanical dysfunction of the sacro-iliac joint. International Rehabilitation Medicine, 1983. 5(1): p. 46-52.
17. Haldeman, S., et al., eds. Principles and Practice of Chiropractic. 3rd ed. 2005, McGraw-Hill.