Spider dragline silk, as a type of high-performance natural fiber, displays a unique combination of tensile strength and extensibility that gives rise to a greater toughness than any other natural or synthetic fiber. In contrast to silkworm silk, spider dragline silk displays a remarkable strain-hardening character for which the origin remains unknown. In this paper, the performance of silkworm silk and spider dragline fibers under stretching is compared based on a combined structural and mechanical analysis. The molecular origin of the strain-hardening of spider silk filaments is addressed in comparison to rubber and Kevlar. Unlike rubber, the occurrence of strain-hardening can be attributed to the unfolding of the intramolecular beta-sheets in spider silk fibrils, which serve as "molecular spindles" to store lengthy molecular chains in space compactly. With the progressive unfolding and alignment of protein during fiber extension, protein backbones and nodes of the molecular network are stretched to support the load. Consequently the dragline filaments become gradually hardened, enabling efficient energy buffering when an abseiling spider escapes from a predator. As distinct from synthetic materials such as rubber (elastomers), this particular structural feature of spider draglines not only enables quick energy absorption, but also efficiently suppresses the drastic oscillation which occurs upon an impact. The mimicking of this strain-hardening character of spider silk will give rise to the design and fabrication of new advanced functional materials with applications in kinetic energy buffering and absorption.