To get a better look at the RF pulse, we have removed the ball representation of the proton and left only its dipole as shown by the orange arrow. The RF pulse comes in from the side, and the magnetic field experienced by the proton from the RF pulse is shown as the green arrow along the Y-axis. Remember, there is still a magnetic field exerted on the proton in the direction of the +Z-axis.
<p>The proton still follows the rules of precession, but the magnetic field it is experiencing has become more complex. It is still precessing around the Z-axis, but it is also precessing because of the magnetic field caused by the RF pulse. Since the Larmor frequency  matches the RF pulse frequency,  the RF pulse peaks along the Y-axis, the magnetic dipole is 90 degrees in the clockwise direction. This is what causes the proton to rotate away from equilibrium in a uniform way. In this graphic, that means the magnetic field from the RF pulse is always pushing the dipole away from us.
If the RF pulse frequency and the proton's Larmor frequency were different, the proton would wobble somewhat but would not rotate away from equilibrium.
It is now important to appreciate these new principles:
1. The rate at which a proton precesses is called its Larmor frequency.
</p>
2. A proton that experiences an RF pulse that has a frequency equal to its Larmor frequency will rotate away from equilibrium.
<a rel="nofollow" class="external text" href="https://www.youtube.com/user/overtone1000">Tyler Moore</a>
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