[tim chapman]

Lag:

The hands leading the club head (Accumulator #2 + 3 lag)
The right elbow leading the right hand (Accumulator #1 lag)
The left shoulder leading the hands (Accumulator #4 lag)
The hips leading the shoulders
The feet should lead the hips

Leading as in being ahead in the rotation.

This is the key to avoid break down of the left wrist and basically anything else too.

When you manage to hold your lag from ground up ...when you turn on the power, the flat left wrist is a consequence of correct lead-lag applications, not a cause. ..
I forgot to mention: You still need a proper power package assembly to make it work. The components must be properly aligned when you start to apply downstroke pressure.

If you're not properly aligned at the top, you will get problems with rhythm and lag pressure in the down swing. You should be able to feel this when it happens. You should be able to feel that your club is is partly escaping the power you try to apply when this happens. A dog chasing its own tail kind of feel.

very helpful Bernt - thank you

took these thoughts, together with 'down/out/forward compression' on the dog walk (with club & balls as usual) this morning & hit a few gooduns