The shell holders just have thicker decks. A standard shell holder deck height above the head of an inserted cartridge case is 0.125 inches. The Competition shell holders come in 0.127, 0.129, 0.131, 0.133, and 0.135 inches of deck height, also called +0.002, +0.004, +0.006, +0.008, and +0.010 inches, respectively. The idea is that if your chamber is bigger than the minimum (and most commercial and military chambers are), these shell holders may be used to set the amount of shoulder setback resizing produces without you having to fiddle the die in and out until you get the desired result. Also, many presses are springy enough that you get more consistent results with the die mouth properly adjusted against a shell holder's deck.
As for the video, I have to say that seems like a lot of trouble to go to. The video author is trying to use the case as a headspace gauge. That is not necessary. Remember, the shell holders come in 0.002" increments, so learning the chamber headspace to the last half thousandth is pointless. Simply fire a few rounds in your chamber, then measure their head-to-shoulder dimensions with a Hornady case comparator. The average of those results should be about a thousandth shorter than your chamber due to spring-back. Start with the +0.010" shell holder and set the resizing die to kiss the raised shell holder plus an eighth of a turn on a steel press, and about 3/8 on an aluminum press after you have lowered the shell holder again. Resize one of the fired cases, looking at it sideways when the press ram is raised to be sure no crack of light shows between the shell holder deck and the mouth of the die (if it does, apply another 1/8 turn until the crack disappears). Measure the case in the comparator again. If it is longer or unchanged, go the +0.008" shell holder and try again. Keep doing this until you find one that results in the shoulder being -0.001 to -0.002 inches below your initial measurement of it. Try it with a couple of more cases to confirm it. So the case is now about 0.002" shorter than your chamber when the setback is combined with the spring-back. This is a good value for bolt rifles. For a self-loader, go to the next shorter deck so you have about -0.002 to -0.004 inches of shoulder setback to insure good feeding.
