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 under his feet. 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.
The portals can be placed in any room. However, if a portal leads to a room with a negative grid index (i.e. Devil Room, Angel Room, I AM ERROR room, Crawl Space, Boss Rush, Black Market, Mega Satan room, or Blue Womb portal room), Isaac will not be able to enter the portal.