Standing still causes Isaac to build up speed at a rate of +0.25 per second, up to a +0.5 speed bonus.
Isaac charges up while standing still. Moving releases a fart cloud behind Isaac, damaging and inflicting Poison on enemies that pass through it and draining the speed buff at a rate of -0.25 speed per second. The fart clouds are left behind for the duration of the speed buff.
The fart clouds deal 0.5× Isaac's damage per tick (1.5× Isaac's damage per second), and last ~2.5 seconds.
Isaac is immune to Poison from enemies, such as Gasbag or Mushrooms.
Notes
As the speed buff requires Isaac to stand still, Jupiter is effectively a speed down.
The fart clouds are mechanically independent of the speed increase. Even while at max speed, the fart cloud must be charged by standing still.
Synergies
Camo Undies: Damaging enemies with the fart clouds will not take away Isaac's camouflage.
Dark Arts: Stays in effect even while using Dark Arts, allowing Isaac to deal more damage to nearby enemies while invulnerable.
Gigante Bean: Greatly increases size of fart clouds.
Tainted Forgotten: Gives a flat +0.20 speed boost and +2 soul hearts with no other effects.
Jupiter, as well as Neptunus, was designed for the original Antibirth mod, but never actually implemented until Repentance.
Jupiter's effects and pickup quote reference the planet Jupiter's nature as a gas giant, being composed mainly of hydrogen and helium in gas and liquid form.
The item's ability to create fart clouds after standing still and poison immunity could be a reference to Nuclear Throne, where the character Frog has similar abilities.
Jupiter is the only item that gives a half-full heart container on pickup.
Grants two empty Red Heart containers and restores half a red heart.
Grants two full Red Heart containers.
Gasbag or Mushrooms.
Camo Undies: Damaging enemies with the fart clouds will not take away Isaac's camouflage.
Dark Arts: Stays in effect even while using Dark Arts, allowing Isaac to deal more damage to nearby enemies while invulnerable.
Gigante Bean: Greatly increases size of fart clouds.
Tainted Forgotten: Gives a flat +0.20 speed boost and +2 soul hearts with no other effects.
Neptunus, was designed for the original Antibirth mod, but never actually implemented until Repentance.
