[O.B.Left]

At issue here is the Plane of the Left Hand Wrist Cock , the Left Arm Flying Wedge.
Both the cup and the arch are Horizontal motions when Homer prescribed Vertical. (although he did mention the arch as insurance against Throwaway).

It's interesting to me that the cup with it's toe pointing down tendency is seen as an open club face, but when you take that cup down to Impact you have a bent Left Wrist, throwaway and a closed clubface.

I don't think the arch is a Secret or anything, a road to nirvana as a general rule. I used to look at it and think it was cool but now I see it as a compensation or idiosyncracy. I'm arched at impact too. But my left thumb just doesn't want to grip the club on the aft. I keep telling it to think of the aft as the top at Fix but it won't listen.

The other thing about that weak left hand grip I'm thinking is that can promote a Slide.