The generation of ultrashort pulses from quantum cascade lasers (QCLs) has proved to be challenging. It has been suggested that the ultrafast electron dynamics of these devices is the limiting factor for modelocking and hence pulse formation. Even so, clear modelocking of terahertz (THz) QCLs has been recently demonstrated but the exact mechanism for pulse generation is not fully understood. Here we demonstrate that the dominant factor necessary for active pulse generation is in fact the synchronization between the propagating electronic modulation and the generated THz pulse in the QCL. By using phase resolved detection of the electric field in QCLs embedded in metal-metal waveguides, we demonstrate that active modelocking requires the phase velocity of the microwave round trip modulation to equal the group velocity of the THz pulse. This allows the THz pulse to propagate in phase with the microwave modulation along the gain medium, permitting short pulse generation. Modelocking was performed on QCLs employing phonon depopulation active regions, permitting coherent detection of large gain bandwidths (500 GHz), and the generation of 11 ps pulses centered around 2.6 THz when the above 'phase-matching' condition is satisfied. This work brings an enhanced understanding of QCL modelocking and will permit new concepts to be explored to generate shorter and more intense pulses from mid-infrared, as well as THz, QCLs.