This exercise strengthens and trains the glutes rather efficiently so if you want well-shaped and round buttock then you have to include this in your training schedule.
Lie down into supine position on the ball, place the weight on your hip with the help of your trainer. In the starting position lower your buttock and hip. If you are placed in the right way and you put your hand gently on the weight (so that it won’t be able to lose balance), lift / push up your hip as high as you can, hold it for a few seconds then lower it to the starting position.
Repeat the exercise with a proper repetition number. In the beginning the weight bar is enough but when you become advanced you may place bigger weights on the bar making the exercise more difficult.