The expectation value of a magnetic moment in a static external magnetic field will precess about the field. The angular frequency of this precession is equal to the gyromagnetic ratio times the magnetic field. If a rotating magnetic field (in the x-y plane) is applied perpendicular to the static magnetic field (in the z-direction), then the magnetic moment will precess about an effective magnetic field in a rotating frame of reference with the same frequency as the rotating magnetic field. By turning on the rotating magnetic field for only a short time, it is possible to rotate the average magnetic moment from the z-direction to the x-y plane. This produces a time varying magnetic field in the x-y plane, which can be observed via an induced EMF in a pick-up coil. The is the basis of magnetic resonance techniques.