The back handspring step-out on balance beam is an intermediate skill necessary for the execution of most high level tumbling skills. These two videos cover the basics of this important skill.
This first video from American Gymnast gives a quick introduction to the back handspring step-out:
In this video, Gymneo.tv presents an excerpt from its very comprehensive video on the back handspring for beam. This 12-minute clip shows pedagogical aspects of coaching this skill. Especially useful are the pointers and drills for staggered hand placement on the beam: