Underwriter's knots keep cords from being pulled from cord plugs. Too often, people just grab a cord and yank it from an outlet without grabbing the cord plug like they are supposed to. This results in ripping the cords from the terminal connections of the cord plug. Plugs and cords that have no built-in strain relief need something to keep them from being pulled out of the connection easily.
An Underwriter's knot is a simple and effective way to support the cord within the plug and give the connection some durability. Cords are often exposed to tugging and being tripped over. In these events, the cord connection often takes a beating and often is pulled out of the plug. With the underwriter's knot in place, the cord virtually cannot be pulled out of the plug.
Install the cap of the plug over the wire before making the knot. Separate about 4" of the two wires and strip the ends of the wires about 3/4" from the end. Take each of the two wires and make a loop over the opposing wire. You simply make a loop in each wire and pull the other wire through that loop. Repeat with the other wire and then pull the knot tight.
Now pull on the wire while holding the plug cap to see if the knot is big enough so that it doesn't pull through the hole in the plug cap. If it is too small and doesn't hold, you may have to add a clamp to the base of the plug cap. Either method will secure the cord from being pulled from the plug connections.