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.
The dash deals damage equal to 4× Isaac's damage + 8.
The direction of the dash can be changed by holding another button.
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.
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.
Curse Rooms and open 
Spiked Chests/ 
Mimic Chests without taking damage.
Shopkeepers.
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.
War when holding the item, referencing the Roman god of war, Mars.
Astrological Signs 
The Binding of Isaac: Rebirth
