Most simple rockets use a solid fuel engine which, once ignited, burns at a steady rate that cannot be controlled or stopped until the fuel is expended. Hybrid engines on the other hand can be turned on and off, and the thrust adjusted by varying how much oxidizer is fed into the ignition chamber to burn with the fuel. The simple hybrid engine made with this experiment lacks any valves that would be found in a full scale model to control the flow of oxygen, as it is the simplest form a hybrid engine could possibly take. You might call it a base model. If one were so inclined, all sorts of modifications could be made to this experiment to allow throttle control and even a nozzle.