As you come toward impact, you want your left side to straighten up. A lot of golfers get the wrong idea; when they're missing shots, they think they need to "stay down," meaning that they collapse their left side. The analogy that I would like you to consider: a field goal kicker. The field goal kicker uses his non-kicking leg as resistance so that his right foot can kick the ball. If he came in and collapsed his front side, he wouldn't be able to kick. A golfer who leans in and collapses his front side doesn't have the necessary leverage either.So as you approach the ball, you plant and straighten your left side like a field goal kicker, so the club goes speeding through the ball.