Taking just one of the experiments, field 0.2 T, frequency modulation (FM) 0.6 MHz, with resonance frequency at about 64 MHz it is to be seen, that when outside the resonant frequency the experiment is simple LTNO. But when the resonance condition is satisfied, the anisotropy of emitted beta-particles drops down.
So some order has just been destroyed. The signal is dropping, so less beta rays are anisotropic – in other words more emitted electrons are randomly oriented. Now the NMR/ON takes place and drop in the signal gives point to determine, that the 91-Sr hyperfine field has negative sign.