I'm an archer. I shoot every weekend.
Modern western style archers are trained to take a solid stance, draw and hold at an "anchor point". The style called "Instinctive" shooting usually anchors the draw hand to the jaw or cheekbone. Another style draws so the string touches the nose. Some use a "kisser button" on their bowstring. They draw until it just touches their lips, every time, to get a consistent draw and release.
Oriental styles use a different method, drawing to a fixed position without a fixed anchor point, often even drawing the hand back behind the head, Their "fixed" point is determined by arm position, usually a point where the shoulder blades meet in back and the arm is as far back as it will go. They call that the "locked" position.
But anchor point or arm lock, both rely on consistent mechanics, to always draw to the same point, always release with exactly the same motion. Don't pluck or roll the bowstring off your fingertips, don't drop your head or shoulder, don't flinch from the snap of the string. Grouping shots is a goal. It demonstrates consistent mechanics. If you can group your shots tightly, even if you're nowhere near the bulls eye, you're good. You can adjust your sight point to move the group until you're right in the center gold.
Ancient archers didn't draw and hold" at all, but drew and released in a single motion. One reason is the "mighty bow" aspect: The bow I shoot is about a 40 pound draw, heavy for some but light by hard-core standards. 50 to 60 pounds is more common among bow hunters and hard core archers.
Those would have been considered children's bows in the middle ages, when bow draws were closer to 150 lbs. If you try to draw and hold a 150 pound bow, your arm gets tired really fast, and your hand begins to shake from the strain. The snap shot, like we're seeing in the video, is actually more accurate at that point.
Grouping shots was less important, and may actually have been a foreign concept, since most shooting was at a live, moving target. Why lock yourself in a stance or solid position when your target isn't in an equally locked position?
For many years competition archers used to fire very light bows, as light as 11 pounds of draw. The idea was to minimize arm strain and hand shake. The arrows merely had to pierce the front of a paper target, after all. Then came a man who worked out, built up his upper body strength, and fired a much heavier bow. His shots flew a very flat, straight trajectory, and never fell out of the targets. It changed the entire approach competitors took.
As for the rapid-shot demonstration in the video: He's not shooting a light bow. His shots punched riveted chain armor, and with little more than target points on the shafts.
Now, would his bow count as "Mighty" in medieval times? Probably not. But it's probably heavier than mine. Even so, his snap-shot style is absolutely appropriate for the heavy bow.
Note that part of the difference in penetration may be the arrow shafts. In a medieval setting, arrows were wood, period. Cedar, ash, bamboo, whatever, they were wood. If they hit armor, the shafts shattered. (D&D mentions a 50% recovery rate for missed shots. That's why.) A bodkin arrow, the long sharp ones depicted in the video, would punch armor effectively enough to let the shaft survive. If the shaft shatters then you don't have the full force of the shot behind that arrow head, and you get less penetration.
Today we shoot carbon fiber, aluminum or titanium arrow shafts. They don't break nearly as easily, so they can deliver more of the bow's power to the target, allowing target points to punch like bodkins of old.
So, as an archer and as an armor maker (I make chain maille stuff to sell at Ren' Fairs), I can appreciate what he's doing there.
