Baryon–meson sum rules enable cross-checks among exclusive mesonic and baryonic measurements. Unlike their heavy-to-heavy counterparts, analogous heavy-to-light relations lack an explicit heavy-quark-symmetry origin. We investigate their origin in the b → sνν¯ system by studying the relevant helicity amplitudes at the two kinematic endpoints. Although the sum rule follows algebraically from the simple short-distance structure, its coefficient depends non-trivially on the hadronic dynamics. At maximum recoil, an approximate large-recoil form-factor symmetry suggests a coefficient of $1/2$, compatible with the numerical results within current form-factor uncertainties. At zero recoil, by contrast, a kinematic helicity projection enforces its exact vanishing. The fully integrated coefficients are numerically close to 1/4. However, unlike the one-quarter coefficient in the heavy-to-heavy system, they are not local symmetry coefficients but integrated quantities shaped by the behavior between the two endpoints. This understanding may guide the construction of baryon–meson sum rules in more general heavy-to-light decays.