In D-T and D-3He plasmas, ICRF heating at the second harmonic of deuterium results in a modification to the distribution function of the heated ions. The authors describe the results of such effects on the dynamic behaviour of plasmas. Using a 0-D plasma transport model, the effect of ICRF heating on an operating point is analysed for ITER and for a D-3He tokamak. To describe the dynamic behaviour, a model for the characteristic time of tail relaxation, τξ, is developed and a feedback model based on auxiliary power is presented. The stabilization of temperature perturbations in a D-3He plasma is simulated for various values of τξ