I was talking by phone with a former NGCI from whom I have taken a couple of lessons. (Scheduling another one.) He said he was going to be checking to see if I was cupping my lead wrist at the top. And suggested I check before the lesson and start trying to feel the proper flat wrist right now, if I was cupping. I said yes, it's been a problem for me the whole time I've used NG/SA swings.
Then he said something that was an AHA! moment for me. If you don't hold the club firmly with the lead hand, the hand will open up a little at the top and allow the wrist to cup. Which of course puts the clubface in the wrong orientation, and requires a regripping during the downswing of all places.
Up till now, I've never heard a reason why the lead hand hold was supposed to be so firm; firm enough to draw blood as Moe said. So I never adopted it. I'm not a compliant student, I guess. I was always one of those kids who said "why why why...."
Anyway, yes, AHA!, my hand was opening up. When I put a firm hold on it, the wrist feels natural in the flat position. When I use my Sam Snead "light" hold, when I manage to keep the wrist flat, it feels bowed and awkward, which my brain rebels against.
A further complication I have found: Over the years of fighting an open clubface, I used a stronger (rotation-wise) grip. (Turned my hands to the rear. The two possible uses of "strong" might be causing problems for any beginning golfers reading this.) I had just watched the most recent free webinar from Graves, presented by Scott Graves. He mentioned how because the club is shafted at the heel, not the center, of the head, that you can feel when the club is properly positioned at the top. And your golf brain will do things to make it feel good. Well, my golf brain wants my lead thumb to be under the shaft at the top, supporting the club. I'm pretty sure that's what he was saying also. SO, when using a strong grip (rotation-wise), in order to support the club comfortably at the top, the wrist has to cup. The stronger the position of the hands, the more cupping needed. A vicious circle.
I'm sure a person can learn to keep a flat wrist with a strongly-turned grip. But it is a fight against the innate golf lizard brain in my case. I think this is going to be huge for my swing. This is the last/biggest hurdle that I've never understood and never overcome in my SA swing. There will be more, but this is a biggie. (Why oh why have I not taken more lessons....)
I bet this was all discussed at length ten years ago in the archives. But maybe worth bringing up again. Keep the lead hand firm, as Moe and many SA instructors have said.