A hand tool with a flat perforated plate or a thick zigzag wire fixed to an upright handle, pressed down repeatedly through boiled potatoes and other soft cooked vegetables to break them into a mash. It leaves a rustic texture with some small lumps, which many cooks prefer to the smoother result of a potato ricer and which is the right texture for crushed potatoes, refried beans, guacamole, apple sauce and the filling for fish cakes.
A blender or food processor is the wrong tool: the fast blades burst the potato's starch cells and turn mashed potatoes to glue. Mash while the potatoes are hot and add warm milk and butter after the first pass.