The firing of a shell sets the "trigger" that trips the ejector spring as the gun is opened. The fired shell(s) are ejected from the chamber and the unfired shell(s) are lifted sufficient by the extractors for removal if so desired. The ejector spring powers a hammer that strikes the end of the extractor, causing it to "kick" the hull out. Note that ejection requires split extractors. Extraction-only can use a single piece extractor that always lifts both shells, but has no "kicker" capability.