1. Researchers at Tokyo Institute of Technology identified that the poor mechanical performance of carbon nanotube (CNT) bundles and yarns is due to localized rearrangements of CNTs during twisting, leading to disclination lines. 2. These disclinations, formed by disrupted hexagonal patterns in CNT bundles, significantly reduce the tensile properties. 3. The study offers insights into improving CNT yarns' performance and could contribute to advancements in computational materials science.