The left wrist works in a vertical plane uncocking and cocking in a "up and down" motion, again VERTICAL. Even when you on the inclinded plane, it's still a vertical motion performed on an inclinded plane.
The right wrist works in a horizontal plane bending and unbending. This is how the left wrist stays flat when the left forearm is turned to the plane...because the right wrist is BENDING on a HORIZONTAL PLANE. Watch Tomasello's video on i think the hands, he demonstrates this clearly.
When you BEND the left wrist you are introducing a VERTICAL MOTION to the RIGHT WRIST which introduces the bend in the left wrist, which also conversely severly opens the clubface.
Take all the "cock" or "vertical" motion out of the right wrist and you'll have a flat left wrist.
Are you saying that ,at the top ,or end,IF YOU COCK THE RIGHT WRIST ,you must bend the left?