Since these eels can't suck in and gulp down their prey as other predatory fish do, they instead evolved this swing-out set of small, grabbing mandibles. The eel first captures its prey with its primary jaws, then swings out its secondary set to grab the fish and pull it down the eel's throat while the first set of jaws chomp down on the next length of flesh.
Check out the great video below as well. Superb. Thanks to UC Davis for this!