The authors give a more extended discussion of the method of 'amplitude correlations' introduced by Crossley et al. (1992) for the study of nonlinear interactions in plasma turbulence, and in particular for determining the sense of energy flow through a turbulent spectrum. For a particular case, the 'frequency doubling' interaction drift waves in the magnetic quadrupole GOLUX, which transfer energy from driving to driven modes, the authors present a rigorous justification of the method: the driven mode amplitude lags that of the driving modes, as the intuitive interpretation would lead one to expect. The authors compare the theory with experimental results, and show that the method can, in principle, lead to estimates of the damping rate of the driven mode and the nonlinear energy throughput rate. A full determination of these quantities, however, requires further information on the dispersion curve of the driven mode, which is obtainable in principle from wave propagation experiments. Finally the authors show that the method requires many fewer data and less computational effort than the conventional method using the bispectrum and bicoherence.