The detection and understanding of the movement of magma at very shallow levels remains one of the most fascinating challenges of modern volcanology, because such information allows us to identify and circumscribe the most probable location where future eruptive vents will open. Unfortunately, it is rarely possible to observe any detail of the internal structure of the feeder system of recent eruptions; in only very few cases, geological observations in dissected volcanoes can help us imagine how magma moved and evolved inside the feeder system. In this paper, we describe the 1809 eruption of Mt. Etna, Italy, which represents one historical and rare case in which it is possible to closely observe the internal structure of the feeder system. This is possible thanks to the presence of two large pit craters located in the middle of the eruptive fracture field that allow studying a section of the shallow feeder system. Along the walls of one of these craters, we analyzed well–exposed cross sections of the uppermost 15–20 m of the feeder system and related volcanic products. Here, we describe the structure, morphology and lithology of this portion of the 1809 feeder system, including the host rock which conditioned the propagation of the dyke, and compare the results with other recent eruptions. Finally, we propose a dynamic model of the magma behavior inside a laterally–propagating feeder dyke, demonstrating how this dynamic triggered important changes in the eruptive style (from effusive/Strombolian to phreatomagmatic) during the same eruption. This is therefore an exceptional case to understand how basaltic magmas move during the propagation of an eruptive fissure, which furnishes fundamental elements for the modeling of superficial intrusive processes. Our results are also useful for hazard assessment related to the development of flank eruptions, potentially the most hazardous type of eruption from basaltic volcanoes in densely urbanized areas.