It grips the flats on a turnbuckle so you can adjust it without twisting the rod ends, which sets toe, camber and link lengths cleanly.

Match it to the flats on your turnbuckles, commonly 4 mm or 5 mm. Check your car, as sizes vary between kits.

Turning by hand twists the rod ends and wears them out. A wrench turns the turnbuckle itself for smooth, precise adjustment.