We discuss the reasons for extremely high wind speed observed in 3D MHD simulations of fast rotating young stars with intense magnetic fields. We find that superradially expanding flux tubes in latitude and in longitude are responsible for a significant acceleration in our simulations. We extend here the analysis presented in Reville et al. (2016) thanks to an analytical model introduced by Kopp & Holzer (1976). We find that the expansion factor observed in the simulations is coherent with the fastest speeds we observe. This phenomena needs to be accounted for to model speed distribution of young stars' winds.