A forming method, in a mold fitted with a wall forming a cavity, of a container from an intermediate container including at least one lateral protrusion projecting toward the exterior of the container. The method includes a boxing operation of pushing the lateral protrusion by an insert movably mounted with respect to the mold between a retracted position in which the insert is retracted into a recess formed in the wall and a deployed position in which the insert protrudes with respect to the wall, in order to form, by overturning, a hollow reserve projecting toward the interior of the container. Also included is an operation to pressurize the intermediate container, including, prior to the boxing operation, injecting a fluid under pressure into the intermediate container; and a local pressurization operation, prior to the boxing operation, of injecting a fluid under pressure into the volume defined in said recess between the lateral protrusion and the insert.