The importance of photochemical reactions proceeding through the formation of spin-correlated radical pairs is becoming increasingly apparent, due to their roles in biological magnetic field sensing and optoelectronic devices. Here we consider the spin-dynamics of triplet born radical pairs under varying weak magnetic field conditions and in doing so provide a simplified model for understanding the low field effect (LFE) by identifying triplet states that cannot undergo coherent mixing to singlet states in zero magnetic field.