Here's a good vid about the bowed left wrist through impact.
Here's one on lag - actually it's an ad for an aid for lag - that's worth watching for the first 1:55 anyway! 
You decide which is the 'problem' you want to address.
Note that in the second vid: 1) a flat left wrist is advocated and 2) the pronate/supinate is not as per anatomy. In anatomy, pronation/supination is a forearm rotation (rolling the wrists!).
Here's a link about lag also. The last paragraph describes a 'drill' too. I actually used an feel reinforcement aid for this last week - where a strangely bern club say on my shoulder at top of back-swing and you had to keep it there for 'longer than currently'!
