When a nucleus experiences two or more states with different chemical properties, it is in exchange. The NMR signal(s) of the nucleus depend on the exchange rate (k), relative to the difference in the chemical shifts of the states. The animations below shows how the spectrum changes with k for a two state exchange with a chemical shift difference of 50 rad s-1. The natural line width was neglected in this animation. At 50/sqr(2), the two peaks show coalescence.
In the animation below, the states A and B are populated 1:1
In the animation below, the states A and B are populated 1:3
Originally from: http://wwwchem.leidenuniv.nl/metprot/ubbink/