Evidence: I first learned this technique from a demo held at the Smithsonian by a Norwegian museum during the Vikings: The North Atlantic Saga Exhibit. They had four already loaded bobbins with wool yarn and had instructions and a kind lady to walk you through it. A team of two made a cord on the spot in less than 10 minutes. We took home our sample cord and a pamphlet on something called Interlocking. Later I found further documentation for this in Margrethe Hald's Ancient Danish Textiles. Lastly I found a webpage by Danr Bjornsson where he made bobbins for his lady to practice this art. The search for more information continues.
Margrethe Hald [p. 240] discusses a leather cord discovered among the Krogens Molle Mose material that could have been made using the whipcord method. Leather cord would have survived better underground than wool. Apparently Scandinavian children still practiced this as a game until recently. One of the main strengths of using this method to produce cording is that you are not limited, either by your arm span or amount of time available, in the length of cording you can make. You can wind extra thread on the bobbins and let them out as you need a longer cord. You can also pause easily and let the bobbins hang and pick it back up at any time. One of the main weaknesses is that it is difficult to use this method for a cording that requires more than 4 threads or loops.
The common thread used during the viking age is wool, as that was readily abundant and fairly easily spun. I have included examples in linen, wool, silk, and a wool-silk blend. The thicker the thread or yarn, the thicker the cording will be. I have used two threads of a lesser diameter on each bobbin to produce a thicker cord.
The bobbins will need to be suspended from something overhead to be at a workable height. I have a planter hook in the ceiling of my sewing room. I have used the rope for attaching walls in the doorway of a pavilion, chandeliers, and pretty much anything stable overhead. I imagine that one could easily use a tree branch overhead, a clamp in a doorway, or any other secure method.
For two workers, each worker holds a bobbin in each hand. Then they diagonally swap the bobbins to create the cord. Right hands switch and then the left hands switch. Lather, rinse, repeat. You swap bobbins as if you were shaking hands, palm to palm. You do not want to swap on the other side, with the back of the hand. You need to make sure that the path is consistent for both hands so that a cord is formed and not two independent sets of entwined threads.
For an individual, hold two bobbins in each hand. Switch them diagonally as with two workers, only keeping the other two bobbins stationary. You want to keep these traveling a consistent path as well. Margrethe Hald describes the one person method fairly well. She has you suspend the bobbins on the right hand, over the back of the hand. The bobbins in the left hand are held by the strings with the thumb and pinkie fingers. You want to swap them, once again, as if you were going to brush your own palms together. With the faceted sides of my bobbins they are easy to manipulate without getting out of control.
Pattern: If you select only two colors to make your cord, you can do two different patterns. To make a striped cord, you each (or each hand) gets a bobbin with one of each color. If you set up so that your right hands have the same color and your left hands have the same color, when you swap the bobbins diagonally you will make a striped cord. For example, light thread in the right hands and dark thread in the left hands will make the striped design. If you want the cord to have a spiral design then one worker (or hand) gets the two of the same color and the other gets the remaining two in a different color. For example, one worker gets the light colored threads and the other gets the dark threads. Then the created cord will appear like a spiral, much like a design achieved with a pattern of fingerloop braiding.
Hald, Margrethe. Ancient Danish Textiles from Bogs and Burials. National Museum of Denmark, Copenhagen, 1980. ISBN 87-480-0312-3.
Pamphlet on Interlocking, Skoletjenesten Vikingeskibsmuseet 1999, scanned in and uploaded at http://genvieve.net/sca/interlocking.pdf
Willadsen, Don. Whipcord Braiding Bobbins by Danr Bjornsson, 2002, http://bjornsson.crosswinds.net/sca/danr_as/bobbins/bobbins.htm