I've read two different explanations so I'm confused. The way I thought NMR works is that the protons spins align with the external magnetic field, then the RF radiation applied causes the spins to flip from -1/2 to 1/2, if the frequency of the radio waves are equal to the energy difference between these two spin states. Now I read a different explanation which has me confused. Here: http://www.simplyphysics.com/page2_4.html it says that the protons actually wobble around the axis of the external magnetic field, and when you apply radio waves with energy equal to the frequency of this wobbling (Larmor frequency), then the protons absorb it. Which is it? Is it the energy difference between the two spin states, or is it the energy of the Larmor frequency? According to this animation: http://www.youtube.com/watch?v=0YBUSOrH0lw&list=PLC40660311CD1C353 the absorbed RF energy causes the magnetic vector axis to drop to the XY plane, so that it is now tangent to the axis of the external magnetic field. This clearly isn't the same thing as a spin flip, so I'm confused.