If there's one skill that I've seen scare gymnasts as they progress in the sport of gymnastics, it's the back handspring step-out on beam.
For most gymnasts, this is the first airborn skill you learn on the beam. First you're standing on the beam, then you jump backwards into the air so that you're not touching the beam, land on your hands, and finally land on your feet, standing upright again. If you think about it, there is a lot of room for error if this skill is not done properly, and injuries have been known to occur.
As with all beam skills, you should begin by practicing back handspring step-outs on a straight line on the floor. This is where it all begins. Everything should be done correctly while practicing on the floor because once you develop bad habits and bring them up to the high beam, they can be difficult to get rid of, and can even be dangerous.
If you already have picked up some bad habits, take the skill back to the floor line, and practice it with the correct technique. Often it will feel strange at first, but it will help you in the long run.
Once you are comfortable with back handsprings on the line, gradually bring them up to higher beams. Advance at your own pace, but don't linger at one stage longer than is necessary. You may wish to get a spot from your coach at each stage to get a feel of the beam, but also be careful not to become dependent on your coach.
Once you get up to the high beam, and are confident that you can do it by yourself, go for it!
Things you should keep in mind:
If you are having trouble getting enough height before landing on your hands, you may not have enough strength in your legs. Try doing the leg squats to help strengthen your quadracep muscles.