Eruptive activity at Etna resumed in the early morning of 14 December 1991, preceded by a 9 month inflation and a seismic swarm of appreciable intensity. In the early hours of 15 December, eruptive vents opened on the western wall of Valle del Bove at about 2300m elevation and fed the main lava flow. The fracturing episode associated with the eruption outburst affected the principal local structural systems, trending NNE-SSW and NNW-SSE, respectively, which in recent years have produced several lava flows. We propose a model of the eruptive mechanism based on a number of seismological, volcanological and field data, discussed in the framework of the available information from ground deformation measurements. In particular, the model hypothesizes the depressurization of a shallow batch of magma, owing to the rapid magma withdrawal, which caused the downdropping of one side of the eruptive fissure by normal faulting. After a momentary stop, a shallow inflation drove the restart of the eruption by the opening of the lower edge of the fissure.