Free throw routines are one of the stranger parts of the game. Player puts up hundreds of shots in a day and doesn't do anything resembling their free throw routine and then all of the sudden they have a separate set of prep movements to shoot a free throw? If anything it makes the most sense to treat it like a catch and shoot situation: Ref passes player the ball, player goes straight into their shot. Anyhow, I don't like it lol
It really doesn't make sense to do that, because every good and great free throw shooter in history knows that they wouldn't shoot as high of a % doing it that way.
What is a must for anyone wanting improve, is to experiment with lots of different routines at the line, including number of dribbles, when and how long you look at the rim, making sure you're on balance holding your release till the ball hits something and not backing away or falling forward.
Also, where exactly you stand at the line. Not everyone is best standing dead center. Ideally, you want to line up to where your release point is centered with the rim, which for most will have the center of your body slightly off center at the line.
Further, free throws are a set shot, which is different than a usual jump shot from that same distance. So where and how you position your feet can be different. For example, many players are better with a more square stance on a set shot at the line