Crystalline Si nanowire (SiNW) springs, produced via a low temperature (<350 degrees C) thin film technology, are ideal building blocks for stretchable electronics. Herein, a novel cyclic crystallographic-index-lowering self-turning and twin dynamics is reported, during a tin-catalyzed in-plane growth of SiNWs, which results in a periodic zigzag SiNW without any external parametric intervention. More interestingly, a unique twin-reflected interlaced crystal-domain structure has been identified for the first time, while in situ and real-time scanning electron microscopy observations reveal a new twin-triggering growth mechanism that is the key to reset a complete zigzag growth cycle. Direct "stress-strain" testing of the SiNW springs demonstrates a large stretchability of 12% under tensile loading, indicating a whole new strategy and capability to engineer mono-like SiNW channels for high performance stretchable electronics.