Gravitation Potion

Drinking it does nothing by itself. It arms a toggle. Press up and the world turns over: you fall toward the ceiling, stand on it, and the entire screen rotates with you rather than just your sprite. Press up again and you drop back. The toggle works in midair, which is the part that matters, because it turns the potion into a way of falling in whichever direction you want. The buff runs on a short clock and you can cancel it early from the buff bar if the orientation is getting you killed rather than helping.

You will find these long before you set up to brew them. They turn up in underground pots, in ordinary chests and in most crate types, which is a steady enough trickle that a long mining session usually leaves you holding a few you never planned for. Since the effect is best spent in short bursts, that trickle often covers everything you need it for.

Flipping wipes the fall distance you have built up, so it looks like a fall damage cure. It is not. Land normally at speed and you take the whole hit. Anything that removes fall damage covers the gap: wings, a Lucky Horseshoe or any balloon built on one, or a Featherfall Potion. With none of those you can still save yourself by flipping rapidly just before you touch down, cancelling your momentum in both directions, but treat that as a panic reflex rather than a plan.

For pure vertical range nothing available at this point competes. Rocket Boots and wings both run out of fuel. This does not, because you are not flying, you are falling. Chain the flips together and you cover an enormous amount of ground for the price of one bottle. That is why it is still the standard way to sweep for floating islands before wings exist: get roughly to the right altitude and bob up and down until an island edge scrolls past.

The Wall of Flesh is the obvious one. Walking the roof of the Underworld puts you above the lava pools, the ruined houses and the elevation changes that normally decide that fight, and in Expert it takes the lava slimes out of the equation as well.

Later, being able to drop upward on command makes Retinazer far more manageable once the lasers start, and it is one of the long standing ways of taking Duke Fishron down well before the mechanical bosses. Bring several. The duration is short enough that one bottle does not cover a full boss fight.

The first seconds after a flip are genuinely disorienting and you will steer into things while your brain catches up. The minimap does not rotate, so lean on it as your reference until the reflex settles. A couple of oddities never invert either. Movement effects like the snow trail from Frost armor keep pointing the old way, and mounts refuse the flip outright, so you cannot ride upside down.