China’s “Double Reduction” policy (2021) expanded adolescents’ discretionary time, creating conditions for greater sports participation. Whether released time becomes sports time, however, depends on mediating mechanisms that remain under-theorized. This paper integrates Becker’s time allocation theory, self-determination theory, and the social-ecological model into a “Time–Space–Behavior” framework that traces policy effects through three layers: time release, space provision, and behavioral motivation. Four structural barriers—supply shortages, motivational substitution, social norm inertia, and evaluation bias—are identified as suppressing translation efficiency. Five research directions are proposed, spanning time-use elasticity, program quality gradients, exam pressure effects, urban–rural disparities, and long-term health outcomes. The paper provides a theoretical architecture and testable hypotheses rather than causal evidence.