You know that stage from Yoshi's Island on SNES where you are shrunk inside the Frog's Belly as a boss?
Yea, pretty much like that. You can't die by being KO'd left or right. The only way you can die is if you go down his digestive hole(hole is in the middle).
You probably will ask how is it possible to die only from a tiny hole at the bottom, but here's how. The left and right walls are bouncy. The more damage you have, the more you will bounce off them. The holes are like blackholes, they suck you in. So at around 150% if you are KO'd and hit the belly wall, you will bounce near the hole and it will suck you in=death. If you hit the throat area,(the pink thing), the belly will shake and the walls will constrict or expand, or the digestive hole will open more, or close. it does different effects. the only platform is the oddly shaped heart in the middle. but no worries if it's too small, you don't die right away if you fall anyway.
again probably hard to imagine but i will have better pic soon