UpisDown flips your gravity in the places where, frankly, normal gravity was getting a little too comfortable.

How it works:

Dig below sea level with solid rock overhead and you'll find yourself pulled toward the ceiling instead of the floor. Walk back to the surface and you'll be unceremoniously returned to the boring downward direction everyone else uses. In the Nether, the flip is permanent — because the Nether didn't need another reason to be hostile, but here we are. The End, remarkably, is the one dimension that keeps its head on straight.

The details that actually matter:

Camera, movement, and mouse input all compensate for the flip so you're not completely helpless. You're only mostly helpless.
Jumping pushes you off the ceiling. Sprint-jumping still works. You're welcome.
Fall damage applies upward. Gravity doesn't care which direction it's humiliating you from.
Diving underwater below sea level triggers the flip after a brief buffer — no accidental inversions from splashing through a puddle.
Nether portals place an obsidian platform near the ceiling on arrival, because landing is your problem now.
The flip animates smoothly. ROLL spins the camera around the Z axis. VERTICAL does a dramatic forward tumble. Both are configurable.

What it isn't:

This is not a full gravity overhaul, not a dimension mod, not a physics sandbox. It's one targeted mechanic applied in specific conditions. Either that sounds interesting to you or it doesn't.