Flat left wrist
 

What causes it to bend? Faulty hand action would be one reason, poor body rotation another?

If you are trying to keep the left wrist flat, the I would say Improper technique.

Lack of extensor action,Improperly set Flying wedges,Maintaining address fix alignments,Overacceleration,Bending the planeline,No release swivel..........can you tell I've had problems!:)

you are all overthinking the issue, mainly it's just the cocking of the right wrist.

Fix that and you'll be fine.

Jim,

The cocking up of the right wrist?

matt

How does right wrist cock relate to left wrist bend,or do you mean flattening of the right?

This tastes a little flat.
 

For starters, following the Vertical Motion of the Uncocking of the Left Wrist with a Horizontal Motion (Flattening) of the Right Wrist.

I get the feeling that if one can achieve the Flat left wrist at impact the rest of the imperatives will fall into place with practice. Anyone else agree?

No, don't agree. Too many golfers out there with a 'fake' Flat Left Wrist (what the heck? :scratch:) at Impact.
Out of the Three Imperatives, if I really had to choose one where if you could do it everything else will fall into place, then I would choose Clubhead Lag Pressure.

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.