Remember the flat left wrist is the EFFECT, NOT THE CAUSE.
But the effect of what you say?
A BENT RIGHT WRIST.
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Focus more on keeping your right wrist bent and your left wrist won't bend, it will stay flat
For me personally, focusing on my left wrist staying flat works better than focusing on my right wrist staying bent. The flat left wrist is the imperative, not the bent right.
I give the focus to the left wrist, too. It also helps me check my grip by seeing what positions the wrists are in at Top. My only concern is that I Hit. On the downstroke my focus goes to PP1 and 3 of my right hand and the forearm. I might be diluting the focus. Or i could be ripe for a bout with paralysis by analysis.