A spindle is a screw jack mechanism which you operate manually using a long handle (sold separately). This mechanism is particularly useful on wider or taller Greenhouses (or for shorter people) where the basic sort of casement stay handle type opener that comes with Action models is difficult to reach. So, it is useful for the Junior Dwarf Wall model, the 3.1m wide Senior and the 3.1m and 3.84m wide Master Greenhouses.