I understand how going through a portal (let's say an un-knot to keep it simple) and then back through it in the opposite direction takes you back where you cam

I understand how going through a portal (let's say an un-knot to keep it simple) and then back through it in the opposite direction takes you back where you came from; it's like passing a piece of string through a ring and then back out the inside again, the ring falls off. But what I don't understand is how going through a portal, then back around the outside and through it again, takes you to the same place. If you run a string through a ring, then around the outside and back through the middle again in the same direction, the ring is stuck on the string.