A damped and driven collective spin system is analyzed by using quantum state diffusion. This approach
allows for a mostly analytical treatment of the investigated nonequilibrium quantum many-body dynamics, which
features a phase transition in the thermodynamical limit. The exact results obtained in this work, which are free of
any finite-size defects, provide a complete understanding of the model. Moreover, the trajectory framework gives
an intuitive picture of the two phases occurring, revealing a spontaneously broken symmetry and allowing for
a qualitative and quantitative characterization of the phases. We determine exact critical exponents, investigate
finite-size scaling, and explain a remarkable nonalgebraic behavior at the transition in terms of torus hopping.