What are the white fibers in the center of a 7-way trailer plug used for?

How can I reattach the plug after accidentally driving away with it still connected? Google isn’t helpful; any mention of fibers brings up fiber optic wire results. Thanks in advance!

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It’s a filler string. You can use it by pushing on it to strip the outside insulation off the cable; it will cut through the outer insulation quite easily. It is so that you do not have to use a razor to strip huge lengths of cable and risk nicking the inner wires.

When making cable, they require a string to draw the wires through particular equipment, so the mechanical tension is on the string rather than the individual conductors themselves.

The white fibers in the center of a 7-way trailer plug are simply a filler string used during the manufacturing process. They are not part of the electrical connection and serve no functional purpose. The filler string is included to provide mechanical tension when the wires are being pulled through the manufacturing equipment, preventing damage to the individual conductors. It can be trimmed flush when re-splicing or repairing the trailer plug, as it has no impact on the electrical operation.