The idea is the same as the Mössbauer spectroscopy, which is to excite the smallest nuclear (hyperfine) levels of a sample and to observe its emission spectrum. The difference is mainly due to the nature of the excitation. Instead of a simple source giving only one energy, we have a very coherent source giving off a range of energies, that cover all possible low-level authorised transitions, but for a very small amount of time. So what we detect afterwards is the natural spontaneous emissions (again on all possible low-level transitions) of our sample, and given that the frequencies are close enough, they will interfer into a beat pattern. From this beat pattern, the hyperfine interaction parameters can be deduced.
The strength of this technic lies in the small time necessary to perform an experience (around several minutes top) and in the “wide” range of excitation frequencies, that excite all possible transitions.