A back flip is easier than a front flip or side flip.
(cf. Back flips are easier than front flips for us )
Because of human body structure and technique of a round off (and handspring).
Therefore, there are some people who can do full-twisting double back on concreate.
Indices of Back flip
Back Flip from Round off(coming soon)
Round off and Back Hand Spring (coming soon)makes your back flip better.