Hollow-Body is a foundational move in gymnastics to teach proper spinal positioning & total body tensioning. The shape resembles a lying banana or hammock. You must flatten your lower back to the floor throughout the drill. It also teaches you how to breathe in a compressed position. Mix between inhaling through the nose into the belly & fully exhaling through the mouth for time or reps.
Start with the tucked body position. Progress to moving the arms & legs away from each other until you’re fully extended. The Hollow-Body Pump, where you move between a tucked & extended position, is a fantastic dynamic variation for abdominal development.
You can also add rocks & rolls to increase the stability demands. Be sure to do it on a padded surface. My favorite variations are doing flys and pullovers with dumbbells. This absolutely crushes your core & mobilizes your shoulders.