I've slept on this now, read the book, meditated on it etc, and here is what I think the solution to this problem is:
The plane of the the left wrist cock is not in the plane of the left palm -
or:
a cocked left wrist is not at a 90 degree angle to a bent left wrist.
Firstly, look at pictures in the book - say 10-6-B #2. The left arm is not in any way on plane, but the left palm is. Per 7-2, the back of he flat left wrist faces down the angle of approach at impact fix - therefore essentially the back of the left hand faces out towards right field (or in my country out towards long off or cover). Now, for simplicities sake, imagine the left arm at low point, where it is perpendicular to the plane line. I think wrist cock at this point would also be perpendicular to the plane line - but because the left wrist/palm faces somewhat to right field this won't be in the plane of the left palm.
This kind of wrist cock gives the look you see in Lynn's videos (I think), and in the pics in the book (e.g. 10-6-B #2), where the left wrist seems to cup a bit at the top (or in fact, cup a bit as the wrist cocks). Basically, it creates an angle which allows the left palm to be on plane while the left arm is not. Note that when you uncock in this sort of plane however, that the left wrist is flat.
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