Metal cups have a connecting chain with which to apply traction. They require a vacuum source. There are two main designs of metal vacuum cups.
The suction tubing attaches to the top of the cup.
The suction tubing attaches to the side of the cup. This allows the cup to be placed further up inside the vagina to allow correct placement over the flexion point when the head is malpositioned. These can be used for rotational deliveries.