Posterior chain is the muscles and fascia along the back of the body, often lengthened and underworked in desk patterns.
Glutes, hamstrings, the back of the calf, and the long line up the spine form one path of pull. When the front shortens, this line is asked to hang rather than to share standing. In 📝Postural / Structural Realignment the work is often to give the front some length so the back can actually work, not to stretch an already long posterior line harder. The chain is a working map, not a dissected organ.
Clients feel it as a back that is tired from hanging, or as glutes they cannot find in a bridge. Practice is a hinge and a stand they can keep without tucking, at a dose they can breathe through.
Sharp unexplained pain is a referral.