Thumb Fighter is a two-player duel between two cartoon thumbs. Each player has a single key that swings their thumb; land hits on the other one until their health runs out. The physics are loose and the hitboxes are generous, which makes it chaotic rather than technical.
It's designed to be handed to someone with no explanation — 'press this key' covers the entire tutorial — and it works exactly as well as that suggests.
One key each. Press it to swing your thumb at the opponent; land enough hits to empty their health bar. Timing beats mashing because a missed swing leaves you open.
Yes, against the computer, though the two-player mode on one keyboard is the point of the game.
Yes — it's one of the lightest games on the site and runs with nothing to install.