Double tapping a direction makes Isaac quickly dash in that direction.
Isaac is immune to damage from all sources during the dash, with the exception of Blood Donation Machines, Devil Beggars, and similar self-damage active items.
At the end of the dash, Isaac gains i-frames that protect him against Blood Donation Machines and Devil Beggars damage. They will also not activate if touched during those i-frames.
The dash deals damage equal to 4× Isaac's damage + 8.
The direction of the dash can be angled by holding another button.
The dash has an approximate 3 second cooldown.
If Isaac collides with a wall or obstacle, he releases 8 columns of fire that deal 10 damage and can burn enemies.
With a controller, tapping the joystick is rather inconsistent, and the dash can often activate by accident. Using the directional pad instead is recommended.
When Mars's dash is charged, Isaac will occasionally flash brightly, and visible steam will emit from his nose.
Extra care needs to be taken with this item, as tapping for precise movements too quickly will cause a dash.
Mars' quality is treated as 1 for the purposes of Bag of Crafting.
Synergies
Camo Undies: Can be used to deal damage without breaking the confusion effect.
Kamikaze!: Kamikaze can be used without taking damage while dashing.
A Pony / White Pony: Lowers the dash cooldown to approximately 1.5 seconds.
Tainted Forgotten: Tainted Forgotten will charge on double-tap if Tainted Soul is not holding him. This can be used to let Tainted Forgotten attack enemies from a resting position, or move him away from damaging hazards so Tainted Soul can pick him up.
Blood Donation Machines, 
Devil Beggars, and similar self-damage active items.
At the end of the dash, Isaac gains i-frames that protect him against 
Curse Rooms and open 
Spiked Chests/
Mimic Chests without taking damage.
Shopkeepers.
Bag of Crafting.
Camo Undies: Can be used to deal damage without breaking the confusion effect.
Kamikaze!: Kamikaze can be used without taking damage while dashing.
A Pony / 
White Pony: Lowers the dash cooldown to approximately 1.5 seconds.
Tainted Forgotten: Tainted Forgotten will charge on double-tap if Tainted Soul is not holding him. This can be used to let Tainted Forgotten attack enemies from a resting position, or move him away from damaging hazards so Tainted Soul can pick him up.
