This one is a little hard to describe, but I'll do my best. Basically, I've found that fall damage for when you hit the ground while flying is not based on the speed which you hit the ground, but rather how long the game thinks you been falling. I've tried using rockets to fly head first into the ground, using Feather Falling IV boot, and it only took off half health.

But here is where things get weird. Fly up high (like to ~256), then dive down to the ground, but also slow down the decent a bit so you aren't hitting the ground fast (but enough to take fall damage). You get OHKOed even though you didn't hit the ground that fast w/Feather Falling IV. It appears the game is judging your fall damage by how long you've been falling as opposed to the actual velocity that you hit the ground.

I'm attached a video demonstrating this. You can tell when I die that I didn't actually hit the ground that fast at all, wheres the first time I slammed myself into the ground with the booster, I didn't die.