In this contribution, we will present a simplified model for thermally driven dynamo developed using the shell technique. One result of the model is the enhancement of global heat transport before polarity reversals, indicating the critical role that heat transport can play in their occurrence. We have clarified this role by analysing a large set of numerical simulations of our dynamo model. In particular, convergent cross-mapping, a test assessing the presence and the strength of the causal connection between two variables in a nonlinear dynamical system, shows that an increase in the convective heat flux can cause the onset of polarity reversals. According to the idea of the critical role played by convective heat transport, the existing two branches of activity observed for low-mass, fast-rotating, fully convective stars may be explained in terms of the efficiency of global heat transport.