Model rocket engines contain a fuel grain to product thrust to propel it forward. When it is ignited, the burning begins from the ignition point outward in a radius from the start where the igniter contacts the grain of the model rocket engine. The flame exhausting through the core in the fuel ignites the core. This initiates a surface area burn that progresses from the core out towards the model rocket engines casing. As the burning surface area of the core increases, the thrust and the burn rate also increases. At the point the fuel is at the radius of the inside diameter of the engine casing, the pressure, burn rate and thrust is at the highest level in the model rocket engine.
Model rocket engines that have an end burning configuration have a spike at the beginning because of the larger initial area created by the burning radius from the ignition point. This spike in thrust is useful in getting a rocket off the launch pad and up to stable speed before it reaches the end of the launch rod. As the burning radius of the fuel increases beyond the inside diameter of the model rocket engine casing, the burn area decreases into the sustaining phase at a lower thrust level until the fuel is exhausted.
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Model rocket engines that are used in upper stages also contain a delay. When the fuel is exhausted, the delay grain begins to burn at a much lower rate. The delay grain is similar to the fuel grain in that it is constructed of a fuel and oxidizer. However, ratio of oxidizer to fuel is lower and other additives are added to produce denser smoke and a slower burn rate in the model rocket engine. This does not produce a significant amount of thrust because of the slower burn. A delay that is designed properly will produce a coasting period and tracking smoke while the rocket coasts to it's apogee. This helps in the recovery the rocket as it is spotted more easily in the recovery phase of the flight.
Model rocket engines that are used at the upper stage of a rocket contain an ejection charge to deploy the recovery system. When the delay is exhausted, the burning surface erupts into the engine ejection charge and ignites it. The model rocket engine ejection charge grains are similar to the fuel grain with the exception that there are many smaller loose grains that have a very larger surface area that burns very rapidly. This rapid burn produces a large volume of gas out of the front of the engines casing where it pushes against the recovery system in the body tube ahead of the engine. The recovery system then deploys out the front of the rocket for a successful recovery and a safe return to the ground.