Quick-release buckles allow you to remove the bottom hardware portion of the lanyard without having to remove the lanyard from around your neck. A very useful feature for keys or for id card swiping systems.
Quick-release buckles come in two varieties: side-squeeze and center push. The actual quick-release hardware is a durable plastic buckle with both male and female parts. The buckle comes apart when either the sides are squeezed or a center button pushed. It can simply be reattached by inserting one side into the other.
One of the two parts of the quick-release buckle is installed at the bottom of the lanyard where the hardware would normally be. Another short segment of webbing is attached to the other side of the buckle on one side and the lanyard hardware on the other side.