Just to jump in here... If you lock out the elbows you are abusing your body and sooner or later it will start to complain.
Keep the elbows slightly flexed to absorb the shock loading... Many thousands of rounds will damage tendons/ligaments if you're locked up.
One of the advantages of the M.I. is that it is "balanced" left to right so you can swing either direction equally well. This is important in fast action shooting.
Some good print resources are:
Matt Burkett's book "The Practical Shooting manual"... It has detailed step by step stuff for posture etc that are helpful for the beginner, and don't encourage bad habits. He laso discusses the level of committment that is required to be a top action shooter in the US. Planning, budgets, training... Its all there.
And for approaching the mental aspects of good action shooting, go to Brian Enos' book "Practical Shooting, Beyond Fundamentals".
Or more physically, take a course from one of the top shooters. Try a GM who offers one, you'll learn more in one day than you can reasonably assimilate.