Tighten the grip release knob (turn clockwise) and flip it down to lock onto the air vent. The two sides of the knob are labeled ‘I’ and ‘II.’ The air vent mounting grip locks with a narrower gap (for thin vent blades) when the knob is flipped with ‘II’ facing outward, and a wider gap (for thick vent blades) with ‘I’ facing outward.