THE LOCKING CLIP
A locking clip is a metal I-shaped piece that is included with most child restraint seats. Most seats have a place on the back or bottom of the seat where you can store the locking clip. When you purchase the seat it may be there or in a separate bag. (Nov. 2015-Seats rarely come with a locking clip any more.)
And some car seats now have built-in locking mechanisms. Be sure to read the car seat manual. If your seat doesn’t have a built-in mechanism and didn’t come with (or you lost) the locking clip, you can call the child seat manufacture to purchase one.
The locking clip is not needed for most car seat installations. The locking clip is needed when the shoulder portion of the vehicle’s lap/shoulder seatbelt cannot be locked to hold the car seat securely. This is the case when your vehicle has emergency locking retractors on the shoulder belts — meaning that the seat belt only locks upon sudden stop or collision — and cannot be switched as in some older vehicles.
However, some vehicles that have emergency locking retractors use locking latchplates to keep the seatbelt tight, so check to see if the seatbelt can be held tight with a locking latchplate before using a locking clip.
You should not use a locking clip if it is not required to properly install the car seat in the vehicle. It is never used for a LATCH installation or a lap-only belt installation.
The other situation where you may use a locking clip is if the seat is tilted to the side when it is appropriately tightened to be snug in the vehicle seat and you are using a lap-shoulder belt in the locked mode of a switchable retractor. In this case you can take the seat belt out of locked mode and reinstall using the locking clip. This way the child restraint will not tilt.
The locking clip is a pre-crash positioning device. It is designed to hold the seat belt tight up to the moment of impact in a crash when the emergency locking retractor of the shoulder belt takes over. It is not uncommon for the locking clip to bend or even break during a crash, but as long as it has held the seat belt in position up to that time, it has done its job.
To use a locking clip, slip both belts (lap and shoulder portion) through one side over and through the other side of the locking clip no more than about an inch away from the latchplate (where the seat belt buckles). You can find instructions for proper use in your child restraint owners manual, though, it cans sometimes be confusing so I recommend finding a local Child Passenger Safety Technician to show you how to correctly use the locking clip.