I think one thing I see most commonly in the pool is that people are not using body rotation properly. This is far worse with back stroke than freestyle. Rotation does two things. One, it gets the top side shoulder out of the water for less drag. Two, it allows you to use a longer lever from your spine to your finger tips. This is what happens if you use just your shoulder to throw a ball, or rotate from your body/spine to throw and the ball goes a lot farther. I am a big fan of the catch up drill. As near as I can tell, it is all about linking the body rotation and kicking together. Side note, the over arm side stroke I swim uses the 'dog paddle' drill for the under side arm, and the top side arm is doing freestyle. Another note, I have found that a slightly concave palm will pull more water than a flat palm. There are reasons for the paddle board paddles and racing skulls paddles being concave.