Those line tests were performed with a berkley digital 50# scale. A split ring was tied to the scale end of a length of line to put the scale hook on. The other end was tied to a small eyebolt on his workbench. The scale either needs to be pulled with a ratchet strap, or better yet a come-along to get a smooth pull. One person can steady the scale and watch the reading counting aloud as the pull increases, while the other person does the pulling and listens to the last number before the pop. We used about 3 ft lengths of line to get a better reading. A short piece is more susceptible to a bad reading because stretch is so minimal. Some lines tend to stretch quite a bit before they break. You can do this to test different lines, or just to test your knot tying ability. As a said before it was interesting how some line nearly always broke at the knot and others just stretched and broke mid-line.