The way I understand it, (and I haven't been wrong today, well maybe once) Their is a really strong spring that is "engaged" by the firing of the shot and the initial recoil. This leaves a "gap" for lack of a better word that allow the lugs to disengage. The inertia of the spring then pushes open the bolt and compresses the "return" spring. I'm sure the "terms" are incorrect but they are descriptive

The Benelli claims to be faster than any other auto shotgun!