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A Gated Buckle (also known as a locking clip) is the composite or metal I-shaped piece that is included with your child's car seat. Most car seats have a place on the back or bottom where the gated buckle/locking clip is stored, but sometimes the locking clip may be in the "extras" bag in the car seat box, so take care not to throw the clip away if it isn't attached to the car seat. Some car seats now have built-in locking mechanisms, so you should check your car seat owners manual to see if your child's car seat offer this feature.

The Gated Buckle is not needed for most car seat installations. It is never used for a LATCH installation or a lap-only belt installation, for example. 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. 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 Gated Buckle. You should not use a Gated Buckle if it is not required to properly install the car seat in the vehicle.

The Gated Buckle is a pre-crash positioning device. It is designed to hold the seatbelt tight up to the moment of impact in a crash, at which point the emergency locking retractor of the shoulder belt takes over to prevent further movement. It is not uncommon for the Gated Buckle to bend or even break during a crash, but as long as it has held the seatbelt in position up to that time, it has still done its job.

If you need to use a Gated Buckle, you can find instructions for proper use in your car seat owners manual. The center bar of the gated buckle always goes underneath the seatbelt, and the locking clip should be placed no more than an inch away from the latchplate. I recommend that parents ask for in-person help from an Authorised Restraint Fitter if they need to use a Gated Buckle.

An excellent online guide to whether or not you need a Gated Buckle, and how to use one properly, can be found at CarSeatSite.com

(modified from the original text courtesy About.com)