Uses: The Arbor Knot is used to attach the fishing line to the “Arbor” or “Spool Center”. In fact the Arbor Knot is really based on a noose knot and, therefore, pulling tightens it. The same knot is used in Bushcraft under the name Canadian Jam Knot where a light rope, e.g., paracord, is being used to compress a load such as a sleeping bag or is used as the first step in creating a lashing.
Tying it: When the Arbor is accessible, the easiest way to tie the Arbor knot is to create a Noose, drop it on the Arbor and pull it to tighten it. If you make a Slip Knot by mistake it will just pull undone. Alternatively, as shown in the animation, first pass the free end around the Arbor or the load; then use the free end to tie the first overhand knot around the line.
Second Overhand Knot: The extra Overhand Knot in the Tag End is tied close beside the first one. It is essential: when the Arbor Knot is tightened the Second Overhand Knot snugs down against the Arbor. Some people recommend tying this Overhand Knot first to make it easier to wrap the line around the Arbor.
Improved Version: Some fishermen wind the loop two or three times around the Arbor before making the first Half Hitch. This increases the friction, which may be useful on a highly polished reel. The direction of these turns is critical. They should be wrapped as though tying a Tensionless Knot: rotating the reel should tighten the wraps.
Releasing: Pulling on the free end, the Second Overhand Knot, loosens the first knot and and makes it easy to release.
Advantages: The Arbor Knot (Canadian Jam Knot) is simple, easily learned and effective.