Another pretty good online instructional:
… The take-off of a back flip is called the “set” or “lift.” To successfully complete a back tuck, you’ll need to learn how to set the right way. This set drill can be used with a spotter (as shown) or onto a stack of high mats …
photo Paula Tribble – Giants GymnasticsLearn a Back Flip – posted by Amy Van Deusen – About.com Gymnastics

