NRS works because of the hyperfine splitting just like mossbauer spectroscopy, but is different with respect to it in that the synchrotron radiation contains a very broad range of energies. This will then in turn make it so that al transitions between nuclear levels are excited simulaneously by the pulses at t=0. The subsecuent decay to the ground state from the diffrent energy levels, all with different lifetimes will result in a superpositioned signal with respect to time, it looks like a beat. From this it is posible to determine the needed hyperfine parameters of the sample.