The hollow body is a fundamental body position in aerial arts. It supports you in a variety of ways, including healthy hang positions. When applied as a precursor to hip keys and inversions, it protects the low back and helps reduce reliance on the lats, drawing strength instead from the core.
To practice on the ground:
Pull the belly button toward the spine to press the low back firmly into the ground, even if there was no space beneath you to start (it's the engagement that matters).
Lift the head and shoulders and look towards your toes.
Lift the legs just slightly off the ground. The lower they are, the harder the core has to work!
*Special tip* Practice "flattening out" your belly to recruit the transversus abdominis as well as the rectus abdominis.
I recommend including this drill in your aerial warm-ups and/or floor workouts. It's also a great option for pre and post backbending.
The hollow hang will be a little different depending on your apparatus, but the key elements remain the same! Keep you core engaged, pelvis tipped forward, and just a slight lift through your legs.