On a bar-to-bar rig hold one fly-bar with each hand, with your legs above you.
There are two principle ways to achieve this:
Firstly, by inverting a regular Cross. From a Cross position, bring your legs up above your head and balance them so your body is upside down.
The second via a Layout. Start a Layout with a Set and a Break. Let go of one hand and twist 90º when your legs are vertical. Catch the second bar and holds both for a moment before swinging back down.
The Inverted Cross is a difficult position to hold for a long time because of the need to balance upside down. While the Layout method is more dynamic, it is harder to get right and therefore makes the position harder to hold.
You can find out more about this trick in my books:
Thanks to Moritz and Nico for providing videos for this page.