Pretend you are pushing open a screendoor that has a horizontal crossbar.
What did you do with your hands?
I'll bet not a one of you said point thumbs up and middle fingers outward to left and right...
Why is it that so many kids do that(?!) when they do backward handsprings?!!
The proper hand position upon landing is one where the thumbs are pointing towards each other - index and middle fingers about 45° inward towards the midline.
The triceps are the active elbow extensors in this skill - and IDEALLY the elbows will be bent (BUT IN THE PROCESS OF EXTENDING) at contact. The elbows should NOT be extended at contact - and of course we do not want the arms acting as "shock absorbers" (elbows bending) as they contact the tumbling surface. A great conditioning exercise for backward handsprings is "diamond" push-ups - make a diamond with your hands - thumbs and index fingers touching - and put your nose in the diamond.
To emphasize hand position - have your gymnasts working on backward handsprings go to a padded wall (or mat leaning against a wall) - chalk their hands up (so they can ***SEE*** their hand pattern) - and practice "punching" with their hands into the wall. PUSHING that screen door open! THAT is the action that is desired.