  Shortly after the discovery of superconductivity in Sr$_2$RuO$_4$,
spin-triplet pairing was proposed and further corroborated by a constant Knight
shift ($K$) across the transition temperature (T$_c$). However, a recent
experiment observed a drop in $K$ at T$_c$ which becomes larger under uniaxial
strain, ruling out several spin-triplet scenarios. Here we show that
inter-orbital spin-triplet pairing features a d-vector that rotates when
uniaxial strain is applied, leading to a larger drop in the spin polarization
perpendicular to the strain direction, distinct from spin-singlet pairing. We
propose that anisotropic spin polarization under strain will ultimately
differentiate triplet vs.~singlet pairing.
