You should read the section on gas safety before attempting any repairs.

The zip tube was used to transfer a flame from the front of the refrigerator to the burner at the rear. It is an aluminum tube with a series of pre-drilled holes along the top of it. Sometimes the zip tube was controlled by a separate gas valve as in the graphic below, while at other times control of the zip tube was incorporated into the main gas control of the refrigerator. In either case, the gas valve for the zip tube was spring loaded, so that when it was released, gas flow to the tube was cut off.

The user depressed the gas valve, allowing gas to enter the zip tube, and lit the gas at the first hole in the tube with a match. Gas would flow through the entire tube and escape at each hole along the tube. The flame from the first hole would ignite the gas at the second hole, which in turn would ignite the gas in the third hole, and so on. This domino effect would cause the flame to "zip" along the tube to the burner. The zip tube would remain lit until the user released the gas valve. See below.

This type of ignition device was necessary for burners such as the Teddington burner and the Klixon valve which needed to be preheated before they would open and allow gas to flow to the burner head. The use of the zip tube did, however, carry over for a short time to the newer style thermocouple safety system which didn't need to be preheated.

Probably the greatest short coming of the zip tube was that it was also an unwanted gas leak detector. Since it sat in the proximity of the other gas controls and connections, the zip tube had the potential of igniting any serious gas leak and singing the user's eyebrows, or worse.

When working with an unfamiliar refrigerator that has a zip tube, or one that has been sitting a long time, it would be very wise to soap down all the components of the immediate gas system to check for leaks. Be careful not to get soap on the zip tube. The soap film can actually plug some of the holes on the tube. This is possible because of the low pressure of propane and the existence of other holes along the zip tube for gas to escape.