I am not an expert either (and if I say anything particularly erroneous please call me out on it), but in general, a heavy warbow requires a stance that is deeper, wider, and involving more muscle groups than your average Victorian-era originated modern archery stance, which primarily focuses on the sport of accuracy at relatively close distances with lighter-weight longbows.
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We don't know exactly how warbow archers used their weapons (as there are none around to show us or an unbroken, living tradition that preserves their teachings, like some traditional archery arts in Native America, the Middle East, and East Asia (granted even these do not stretch unbroken to the Medieval period)), though we have a few hints here and there in period illustrations and in rare Renaissance-era archery texts. I'm amazed at how much more material there is out there for close combat of the same period as the warbow than for ranged ballistics, honestly, though it's hard to learn archery from just a book, and most warbow archers wouldn't necessarily have been from the most literate class of people.
The rolling loose mentioned is a modern method (afaik) designed to gain some range when shooting long distance, a common goal for warbow archers today (and as a sport back then, as targets/buttes could be as much as 300-400 yards away and the closer ones reserved for military use (afaik) !) as distance and long-range "accuracy" is a relatively easy way to measure the power of a shot. Again, look it up or ask others for more info, I'm not the one to consult with there.
Also, English period literature, when it mentions warbow archery, uses the phrase "bending the bow" as opposed to "drawing" it, which was more continental Europe (the "Flemish" style being drawing with only two fingers; they must have had strong fingers or lighter bows to do that!). You are pushing into the bow just as much if not more than drawing it back to your anchor point (which for a warbow arrow is farther back; it's what, usually on average 30.5-32" or the length of the arrow so that you draw it back to the base of the forged tip?)
I personally think that the Medieval/Renaissance archery stance and technique varied greatly from region to region and, inspired by period art, resembled fencing footwork/stance at the time more so than the modern archery stance with both feet at a 90 degree angle to the target. One thing you see in artwork is that their front foot is often facing forward (anyone want to chime in on that?), though this may be an element of Medieval illustration rather than realism; difficult to say. I also find some mentioning in the period of "stepping" while drawing, using your legs and hips to gain further strength in the draw and range in the shot, though Roger Ascham later berates archers of his time for "hopping around" too much.
Anyway, to summarize:
-longer draw length (the fullest length of the arrow drawn as possible, usually at least to your ear or past it)
-more body muscles involved in the draw (various methods: by bending into the bow, rolling loose, stepping method, being really strong like fishfinder
, or whatever works for you, etc.)
-slightly different stances than modern versions to account for different needs
I hope that is helpful, and anyone who knows better/more than I do about things I mentioned or didn't please chime in.
