One of the controversies among more traditional orthopedic practitioners and the manual medicine community is SI joint movement.  How can a joint that is held together with strong ligaments move?  The SI does move, but not like the elbow or knee.  These hinge joints were built to move large amounts.  The SI on the other hand is a viscoelastic structure like silly putty that was built to stretch small amounts (like a shock absorber).  When you walk, the short duration forces being placed across it by you foot hitting the ground and the bending of the spine cause it to stretch and store energy.  This energy is released by swinging your leg forward.   However, sitting causes some lengthening and stretching of the ligament (like a paperback book placed on top of silly putty).