From the 2015 Junior Worlds Training Hall …
Great exercise that combines strength with flexibility training.
Note how the back stays perfectly flat at all times (for the ladies, not so much for the guys in the video 😀 )
Update: Lydia Valentin also tried them!
Update: Chen Xiaoting doing them in the 2016 Asian championships training hall.
Update: Here is a good progression you can use if you can’t yet get into the position.