HollowBack

Practise on the wall first. Stand about 1 metre away, just enough so it's one full straight kick distance... place your hands on the floor, about shoulder width apart, and jump forward. Kinda scary when you start, so you can stand closer to the wall if you want, but for proper training, 1 metre at least. Put a matress or something if you want, or wear a helmet incase you slipped.

Keep your arms straight, tuck your chin in, and relax your shoulder. Straigthen your back as much as possible if you can. Remember to breathe out, don't hold your breath. When you slip or fall down, don't throw your hands up like a lot of people do. Instead, just lower your legs before your butt so they'd land first before your back. Try to throw yourself to the side too so you don't land on your spine, or worse, head.

It's better to learn unsupported handstand first before hollow back.