Maximum stability in this position would be if the heels are pulled all the way to the po, but then the angle of the feet is no longer suitable for the balance. It is important to find a good compromise. The back is always under tension … no position for having a rest.
The possibilities in this position are very limited, since even small shearing forces mean the end of the trick. The base can not help in case of a crash.
It makes sense to initially balance in this position only a lying flyer.
This position looks very nice in the middle of a pyramid, but is barely reproducible and has never been shown in a performance to my knowledge. May be that this is why some acrobats have dubbed this trick the “birth” …
From an aesthetic point of view, this position does not give much, so it is more of a position for strength acrobats. Here is probably the most famous duo of Cirque du Soleil, at which the flyer holds a handstand on the feet of the base → the Alexis Brothers. Entrance and exit of this trick require considerable strength of the base in the back leg muscles and a flyer that could balance his handstand on a slackline …
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