In order to understand Ramsey's NMR technique (see picture), first imagine the neutron with its spin axis upright in a vertical magnetic field. A very small horizontal magnetic field that is oscillating at just the right frequency will make the spin axis slowly tip over. However, rather than turning it upside down all in one go, it is done in two short 'bursts'. In between these bursts, the spin axis is left to precess around in the horizontal plane, while, in perfect synchronisation, the 'clock' that produces the oscillating magnetic field ticks along in the background, waiting to be allowed to produce the second burst. Now, the trick here is that if the 'clock' is running either slightly more slowly or slightly faster than neutron spins are precessing around, then during the long period between the two bursts of oscillating field, the two systems will get more and more out of step. If that happens, then the second burst won't be all that efficient at tipping the spin axis downwards any more. In fact, if the neutron spins are 180 degrees out of step with the clock by the time the second burst arrives, the spins will be tipped upwards again instead of downwards! So, by simply counting the number of neutrons whose spins are tipped downwards, and comparing with those whose spins end up pointing upwards, it is possible to see very small differences in frequency between the neutron spin precession rate and the clock that produces the oscillating field.