Zip Kick is Robin's version of the Batclaw. Press 2 twice quickly to use in combat. It has no effect on shielded thugs.
If you don't have it yet, during the predator battle with Harley, thugs will keep spawning to ensure she has two at all times. Lure one into a corner, drop behind him, zip kick, then ground takedown him and flee. Repeat five times to get the achievement.