Spider dragline silk is a semicrystalline protein polymer in which alanine-rich crystalline regions are connected by soft glycine-rich linkers that, in dry fibers are quiescent but become motionally active when plasticized by water. Using solid-state NMR to probe molecular motion and orientation of selectively C-13-labeled glycine residues, we observe a collapse of amino acid chains in hydration-induced local phase transitions and find that the relative abundance of the static and mobile phases can be controlled by fiber strain. Our results suggest a molecular mechanism for silk's mechanical properties based on latent entropic springs that are drawn in preformed extended structures, stabilized by interchain hydrogen bonding, and collapsed by hydration.