A classical mechanical analysis of unimolecular reaction is presented, focusing on two- and three-mode models of the dissociation of hydrazoic acid, HNNN, along the central N-N bond. Our previous studies of the six-mode dynamics of this reaction revealed that activation of the HN-NN stretching vibration reaction coordinate occurs via a sudden, large scale, aperiodic, "impulsive" transfer of energy. This transfer results from very specific dynamics, in which the phase of the HNN-N stretch is carefully correlated with the approach of the HN-NN stretch to its last inner turning point before reaction. In this paper, we develop a more detailed understanding of these dynamics. Our two-mode model of this reaction, which contains the HN-NN and HNN-N stretches, reproduces the essential behavior of the six-mode model, indicating that only those two modes are necessary for reaction. Examining the phase space of the trajectories just prior to crossing the transition state reveals that all trajectories enter a "reactive cylinder", as described by De Leon and co-workers, which leads through a region of large coupling and thus produces the impulse of energy into the reaction coordinate. Viewed in reverse time, the path of the cylinder as it leaves the transition state is governed by the near adiabaticity of the reactive mode during the last half-vibrational period before reaction, producing a convergence of phase in the nonreactive HNN-N stretch. Our study of the three-mode model of this reaction, which includes the H-NNN stretch, reveals that there is no such constraint on the motions of the H-NNN stretch.