I think the answer to the question "How flat is flat?" can be found in the right wrist bend at impact fix. In this location the right arm flying wedge includes a precise amount of right wrist bend. Your left wrist will appear flat and vertical or your hand will have some degree of turn on the grip. As long as the right arm flying wedge with its fixed degree of right wrist bend is maintained, then you'll have a geometrically flat left wrist.
Backstroke visual check (swinger) is straight foreword. The flying wedges are assembled (swinger) at the end of the takeaway, aproximately two feet back. The right arm flying wedge is completed and includes its precise amount of right wrist bend. At this position the amount of visual left wrist bend is only the result of a turned left hand at impact fix. This visual amount of bend should exist at the end of the backstroke. Attempting to flatten the left wrist to comply with visual flatness will affect the wedges.
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