[en] A model which includes toroidal coupling between the 3/1 mode and an interior 2/1 mode, as well as coupling to an externally applied (or stray) resonant field, is able to explain the observations in this study. The inter-mode coupling causes rotation of both magnetic mode structures at the same (Mirnov) frequency but also high-frequency difference oscillations of small amplitude. In addition, bouncing of the combined 2/1 and 3/1 mode structure (or the 3/1 alone) in the potential well of the external field can occur after locking. The 'vibration' or 'bounce' frequencies are the only discrete features and therefore provide a potential diagnostic for studying precursor growth in locked-mode major disruptions. (author)