Yeah, green fertiliser, I missed that, despite doing it myself ! you grow "field beans" as a crop then "dig in the tops without removing the roots" - at least on the market garden / allotment scale.
On the field scale, I think you just harvest and plough in after the plants have died back, it is a productive alternative to a leaving a field fallow. On a mixed farm you put livestock in to graze the tops directly then the manure produced can also be ploughed in, which gives the land "more heart" - according to family lore.