flat wrist for me made a huge difference all by itself. But the rest of this post is important as well... except for the "extensor action" ... i don't even know what that is 
divot dave,
Check extensor action out, there are numerous articles on it (mostly from TGM people) it has some very good things going for it, gives the arms some solid stucture, removes any arm over-run, really helps in maintaining a flat left wrist, oh yeh it also gives great extension through impact and beyond.
Feels pretty odd to start with (I think most people stiffen the left arm to try and help, but thats almost the opposite of what you need to feel), but once you figure out the amount of force required, and the way in which you need to apply it, it really is simple and adds some very nice elements to the swing.
I'd direct you to some info but most of it is from TGM disciples, and I'd rather not inflict that on someone, so have a good search about I'm sure you'll find what you need, maybe some of the pros on here have some info or input on it.
cheers
