  Using a dynamic variant of the Vicsek model, we show that emergence of a
crush from an orderly moving human crowd is a non-equilibrium first order phase
transition. We also show that this transition can be reversed by modifying the
dynamics of a few people, deemed as game changers. Surprisingly, the optimal
placement of these game changers is found to be in regions of maximum local
crowd speed. The presence of such game changers is effective owing to the
discontinuous nature of the underlying phase transition. Thus our generic
approach provides (i) strategies to delay crush formation and (ii) paths to
recover from a crush, two aspects that are of paramount importance in
maintaining safety of mass gatherings of people.
