The lat pull down targets many muscle groups. It primarily works the latissimus dorsi, which are your lower back muscles. In addition it also works your biceps, your delts (upper back) and more. So it definitely a great asset to your exercises.
The key with this exercise it to lean slightly back and keep your back straight, your arms should be the only thing moving with the exception of twisting your body slightly back as you pull the bar all the way down.
If the bar is causing your body to sway as you go up or down decrease the amount of weight for better control.
The reason you add the extra twist at the end is because it draws your arm just a little bit further behind you which will work those lower back muscles even more.