J-Coupling

When spin A of a nucleus is coupled to a spin B, it can 'sense' the position of that spin in the B0 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