**J-Coupling**

When spin A of a nucleus is coupled to a spin B, it can 'sense' the position of
that spin in the B_{0} field. If spin B is with the field, spin A
resonates at a frequency different from when spin B is against the field. The
population of A spins is therefore split into two. The red magnetisation vector
represents one of these sub-populations and runs faster than the rotating frame;
the blue one represents the other population and runs more slowly. The frequency
difference between the vectors is the J-coupling. In 1/J seconds, each vector
turns 180 degrees and they meet at the -Y axis.

Originally from: http://wwwchem.leidenuniv.nl/metprot/ubbink/NMRanim_Jcoup.htm