The origin of these cross-peaks can be understood through analogy with non-equilibrium stopped-flow NMR. Following Christianson and Landis, we consider first only 'A' spins present at the beginning of the t 1 evolution period, and work in the rotating frame of spin B. As the A spins precess, those that chemically exchange to state B have zero frequency in this frame and do not precess further (barring further chemical exchange). In other words, the A magnetisation vector can be envisaged as 'dropping' B spins behind it as it precesses (Fig. , Supporting Movies 1 and 2). As illustrated by this schematic, the total magnetisation of these B spins, M B , precesses with offset Ω A = ω A ω B about a point displaced from the origin. This can be represented (Fig. ) as the vector sum of magnetisation, M B , along +y, and magnetisation, M B , that is initially along y and precesses with offset Ω A .