The pliers have cups in each jaw to hold the hog ring as well as a tension compression spring that holds a hog ring firmly in the jaws until it is closed, leaving one hand free to hold material in position and the other holding the pliers
Hog-ring pliers were originally made to attach ear tags to hogs but quickly became common in marine and auto upholstery.
Also useful for gardening, fence repair, attaching netting to support wires, putting winter shields on shrubs, closing bulk storage bags, etc.
NOTE: The part number for the hog ring clips is
U-HR.
Hog Ring Pliers