It’s not a bellyflop competition, it’s called dødsing which kinda translates do “deathing”. The idea is that you spread out as you would for a bellyflop, but tuck into a sort of folded-in-half-dive right before hitting the water. The contestants are scored based on style and how long they wait before tucking in. I’m pretty sure a full-blown bellyflop from that height (10m) is potentially very harmfull. As the name implies it’s about narrowly avoiding death.