As discussed earlier, an rf pulse has the effect of tipping the magnetization from the +z axis through an angle q so that its projection can be viewed in the x-y plane. At some time after the pulse, the magnetization has returned to equilibrium, by the process called spin-lattice, or T₁, relaxation. But what happens in the meantime? This is not an idle question, because the NMR signal is actually recorded during this interval.

If a nucleus were precessing at exactly the Larmor frequency, we would see it decay smoothly from a maximum, right after the pulse, back to the starting position. The intensity would be a plot of the decrease in the size of the projection (shadow) of the magnetization as it lifts out of the x-y plane. The observed signal would look like the curve shown below; this is referred to as the on-resonance condition: