I will admit, it is difficult to keep the left arm straight in the back swing. About half way through the bucket I realized that I started bending my left arm. The bend was not as prominent as it was initially before I started keeping my left arm straight. I had to get used to shifting my weight too. My weight was shifting on my back foot more than it should have, causing me to almost lean back on my swing. If you feel yourself not stationary or not moving forward doing your swing then you might be putting too much weight on your back foot.
Initial post: http://golfwithgoogle.com/?p=53