Boost-sustain solid propellant rocket motors have been used to meet the demand for short and medium range tactical missiles. These motors obtain a wide variety of thrust-time profiles if the propellant characteristics and grain geometry are properly selected. These Dual Thrust Rocket Motors (DTRM) can have various designs; the DTRM with an intermediate nozzle is one design that yields a high boost-sustain thrust ratio with a stable operation. Under certain conditions, the flow through the intermediate nozzle at the start of motor operation can be reversed. The present paper is intended to understand the phenomena and determine the operating parameters of DTRM with the reverse flow intermediate nozzle. In this study, a mathematical model describing the performance is derived from the main governing equations of internal ballistics (IB). Also, a set of experiments are conducted so as to inspect the details of the physics of such configuration DTRM. The experimental findings are utilized to validate the mathematical model. Results of the theoretical and experimental investigation explain most of the IB characteristics that were not discussed before in the open literature.