Growing silicon nanowires (SiNWs) into precise locations is a key enabling technology for SiNW-based device applications. This can be achieved via in-plane growth of SiNWs along a simple step-edge, where metal catalyst droplets absorb an amorphous Si matrix to produce c-SiNWs. However, a comprehensive understanding of this phenomenon is still lacking. We here establish an analytical model to address the driving force that dictates the growth dynamics under various droplet-step contact configurations, and to identify the key control parameters for effective guided growth. These new principles were verified against a series of experimental observations and proved to be powerful in designing a stable guided growth configuration. Furthermore, we propose and demonstrate a unique ability to achieve in situ capturing, guiding and release of the in-plane SiNWs along curved step-edges. We suggest that such a new understanding and results provide a fundamental basis and a practical guide for positioning and integrating self-assembled nanowires in a wide variety of material systems.