When used for the first time, Isaac creates a blue portal under where he is standing. The next time he uses the item, he creates an orange portal. Isaac can then walk over either of the portals to teleport instantly to the location of the other one.
Activating the item more than twice will replace the older of the two portals.
Upon entering a new floor, the previously placed portals will disappear.
Upon teleportation, Isaac is granted with a short invulnerability period which can be abused to trigger the Blood Donation Machine and Devil Beggar without losing health by placing a portal just next to it, teleporting, and touching the Donation Machine or Beggar in the same move. This trick is particularly useful for The Lost. The teleportation trick can also be used to enter a Curse Room without taking damage.
Upon teleportation, Isaac is granted with a short invulnerability period which can be used to enter a Curse Room without taking damage. However, it cannot be used to play Blood Donation Machines or similar machines for free. Isaac is also unable to open chests or pick up consumables and trinkets during the animation, meaning Spiked Chests cannot be opened for free.
Portals will stay even if the item is dropped or changed.
It is possible to stack the two portals on top of each other. Entering one will make you hop infinitely as long as you stay on the portals. It creates invincibility at the cost of mobility and almost, but not all tears.
If Curse of the Maze is active, using portals placed between different rooms may result in a random destination. Two portals in the same room work reliably.
If room contents are randomized by the curse, it's possible for the portals to end up completely surrounded or even inside rocks or pits. If Isaac can't move to safety, the run can be quit and resumed to be placed at a door.
On Downpour II, Ventricle Razor cannot be used to teleport between the normal and mirror world.
This item can be useful to exit Mom's boss room to start the Ascent, as long as you use it at least once before entering the boss room.
The item only costs 10 cents when found in a Shop.
Synergies
Book of Virtues: Spawns a blue/orange wisp that circles the respective portal. Wisps do not fire tears. Wisps do persist if Isaac leaves the room and reenters.
Tainted Magdalene: She can abuse the invincibility from the stacked portals to let enemies approach her and die in her hugs[1].
Interactions
Car Battery: Renders the item conventionally useless except for the unlimited invicibilty described above.
Devil Room, 
Angel Room, 
I AM ERROR room, 
Crawl Space, 
Boss Rush, 
Black Market, 
Mega Satan room, or 
??? portal room), Isaac will not be able to enter the portal.
Challenge Room or the 
Upon teleportation, Isaac is granted with a short invulnerability period which can be abused to trigger the 
Blood Donation Machine and 
Devil Beggar without losing health by placing a portal just next to it, teleporting, and touching the Donation Machine or Beggar in the same move. This trick is particularly useful for 
The Lost. The teleportation trick can also be used to enter a 
Curse Room without taking damage.
Upon teleportation, Isaac is granted with a short invulnerability period which can be used to enter a 
Spiked Chests cannot be opened for free.
Curse of the Maze is active, using portals placed between different rooms may result in a random destination. Two portals in the same room work reliably.
Downpour II, Ventricle Razor cannot be used to teleport between the normal and mirror world.
Shop.
Book of Virtues: Spawns a blue/orange wisp that circles the respective portal. Wisps do not fire tears. Wisps do persist if Isaac leaves the room and reenters.
Tainted Magdalene: She can abuse the invincibility from the stacked portals to let enemies approach her and die in her hugs[1].
Car Battery: Renders the item conventionally useless except for the unlimited invicibilty described above.
Ventricle Razor works normally.
Bug!
Found Soul familiar to become invisible until Isaac drops and picks up the trinket again or enters a new floor.
The Binding of Isaac: Rebirth
