Recoil

Martin Magni's Recoil removes walking and jumping entirely, the only way to move is firing your weapon and letting the recoil push you the opposite direction, which turns every shot into a movement decision as much as an attack against the pink viruses in your way.

Because firing is both offense and locomotion, shooting away from a direction you actually need to travel costs you progress rather than just ammo, reading a room's layout before firing tends to matter more than reacting to a virus on sight. Chaining shots off walls and ceilings to redirect momentum is often the only way past spike-lined gaps that a single straight recoil can't clear. Flip Bros runs a similarly unconventional recoil-and-momentum platforming format.