These are like giant jaws suspended from a crane. They can take big bites of dry bulk cargo such as wheat, coal, woodchips or minerals. The crane holding the bulkgraps can swing the load around, open the jaws and drop the contents.
Sometimes the grabs are covered to limit dust from escaping. Sometimes the grabs need to be lowered into holds to avoid the wind blowing dust all over the place.
Often when the grabs are not able to remove all the cargo, it is removed from the ship's hold using a large suction pipe or a machine with a screw auger.