Split ring pliers typically have a small, pointed surface that fits into the gap in a split ring, allowing you to open it without damage. That provides the ability to slip off the old hook by rotating the ring in the appropriate direction. Then you can replace it with a new one by reversing the process.   Split ring pliers can be used to remove swivels from lures, and blades from spinnerbaits. But most often they’re employed to change out hooks; either one treble for another, or a single in the place of a treble.