An NMR Experiment

resonant condition: frequency (w1) of B1 matches Larmor frequency (wo) energy is absorbed and population of  a and b  states are perturbed.

Mo

B1 off… (or off-resonance)

B1

Mxy

w1

w1

And/Or:  Mo now precesses about B1 (similar to Bo) for as long as the B1 field is applied.

Again, keep in mind that individual spins flipped up or down (a single quanta), but Mo can have a continuous variation.

Right-hand rule