During a recent reversed toroidal field (BT) campaign at JET, experiments were performed to investigate the effect on sawteeth of NBI-driven toroidal plasma rotation counter to the direction of the toroidal plasma current and BT. A power scan at constant density has permitted analytical continuation, into the reverse BT domain, of previous experiments with forward field and hence co-rotation. Earlier JET results were confirmed indicating that counter-NBI injection results in shorter sawtooth periods than in the Ohmic regime. This study has demonstrated that with counter-NBI the sawtooth period has a minimum at 4MW, whereas with co-NBI the period increases with power. Clearly this trend also manifests itself in terms of the toroidal plasma rotation, for which a minimum is observed for counter-rotation frequency ~ 2kHz. The kinetic effect of trapped thermal ions on the internal kink mode is identified as mechanism sensitive to the sign and magnitude of the toroidal rotation, qualitatively consistent with the main features of these observations.