Low wrist means that the pressure point, when drawing the bow, lies in your palm and not between thumb and index as it would when shooting a recurve.
Fred Asbell wrote in one of his books that a longbow is shot preferably with a firm grip. He says that you have to control the bow or the bow will control you. I totaly agree with him on that point, my results are way more constant if I shoot with a grip that is firm and not to loose.
I don't like a narrow grip, therefore I make the grips on my longbows a bit wider than usual and shape a small flat surface for my palm to rest on. By doing this I make sure that the bow lies flat and stable in the bow hand and don't wiggle sideways during the draw.
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Andy