The photoluminescence (PL) of momentum-indirect $\mathrm{\Gamma }-K$ excitons in monolayer ${\mathrm{WS}}_2$ under biaxial strain was recently observed by [Blundo et al., Phys. Rev. Lett. 129, 067402 (2022)], yet the microscopic mechanism governing their formation, optical activation, and valley response remains unresolved. Here, we establish a unified microscopic framework bridging exciton formation, population redistribution, PL, and valley polarization. We identify a previously overlooked higher-energy intermediate exciton and reveal a two-step formation mechanism: Optically generated bright excitons first transfer to this intermediate state through exchange interaction and then relax into momentum-indirect excitons via spin-flip scattering. This mechanism explains the strain-dependent PL redistribution in experiment and identifies true hybridized direct-indirect eigenbranches as the PL-active states. Beyond explaining experiment, our theory predicts a strain-driven, sign-changing “sinusoidal” valley response arising from competition among intermediate-state-mediated relaxation channels. These results provide a route to strain-controlled manipulation of long-lived momentum-indirect valley excitons, opening a pathway toward strain-programmable valley engineering in two-dimensional excitonic systems.

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• Excitons
• Strain
• Valley degrees of freedom
• Valleytronics