Conventional neutron spectroscopic methods (3-axis, TOF, backscattering) define the incident energy Ei and final neutron energy Ef separately. Both energy selections have to be done with the desired resolution of the instrument. Therefore, the detected neutron flux decreases as the square of the resolution. This makes it impossible to reach resolutions below about 1 µeV without prohibitive loss of counting statistics. One basic idea of the NSE spectrometer is that only the energy transfer-the difference Ef-Ei-has to be known. So a large range of incident neutron energies (e.g. 20% spread) can be used provided that the neutrons carry individual information of their energy. This is achieved by the second idea of NSE to use the neutron spin's Larmor precession as an internal clock.	Larmor precession The neutron owns a spin of 1/2 and in consequence a magnetic moment (P). When put into a magnetic field (B) the spin will start a precession movement around the magnetic field direction called the Larmor precession. This rotation has a fixed frequency which depends only on the magnetic field applied. It is therefore possible by applying magnetic fields of definite strength and duration to perform controlled manipulations of the neutron beam polarisation.
In practice, the neutrons are guided through identical magnetic fields before and after the scattering. A πflipper close to the sample position inverts the spin orientation so that the second rotation exactly compensates the first if the speed of the neutrons is not changed by the scattering (elastic).
In addition, π/2 flippers are used to convert longitudinal into transverse polarisation and vice versa. So the spin orientation (polarisation) is restored at the end of the instrument what is called the spin echo.
If the scattering is inelastic, the neutrons stay either longer or shorter in the second field after scattering. Then, the rotations do not match and the polarisation is (partially) lost. So from the decay of polarisation it is possible to deduce the inelastic scattering in terms of the intermediate scattering function S(Q,t). See movies here for the cases of elastic and inelastic scattering in the NSE spectrometer.
Reference: F. Mezei (Ed.), Neutron spin echo: proceedings of a Laue-Langevin Institut workshop, Lecture Notes on Physics 128, Springer, Berlin, 1980