Geometry Vibes X-Ball is a wave-style rhythm game: hold to fly upward at an angle and release to dive, threading a ball through narrow gaps in time with the music. It also supports two and three players sharing a keyboard, which is unusual for the genre.
The multiplayer changes it completely — the same level with three balls to keep alive is a very different problem.
Alternate rapid short holds and releases. There's no neutral input — you're always rising or falling — so level flight is a rhythm rather than a state.
Yes — each player controls their own ball on the same level with their own key, which makes it considerably harder and more fun.
Yes — a light 2D game with nothing to install.