Swapping is a two-pronged process by which people could theoretically travel between First Realms by using the Second Realm as a sort of bridge. This method of "travel" is sometimes used by Aeternalae to explore other First Realms through a form of lucid dreaming. Though when a person travels between alternate realities outside of dreaming however, the least understood aspect of Swapping could occur, thus theoretically bypassing the enigmatic Second Gate. This theory had been pioneered by Kerina with the help of Millie. Testing this theory using both ningen and Ovaform subjects have yielded mixed results, as a solid connection between oneself and their otherworld counterpart (referred to in this article as an "alternia") is required and not easy to attain. Though once such a connection is acquired, anyone can use this method of travel, including Ningen.
The process of Swapping is aptly named because during this process, the spirits of that person and their alternia literally swap bodies (and therefore worlds) with eachother and is the only known way to reach other First Realms aside from dreaming. When swapping occurs, there is a tendency for a person to leave residual memories behind in their original bodies, making it easy for an alternia to pose as their counterparts when the need arises. The amount of memories and the subjects of these memories that get left behind varies greatly from person to person, some more than others. Their alternias can pick up on these memories when their in the body of their counterpart, but in turn they forget various things about themselves and their homeworld for as long as they're in that body.
To put it in more simpler terms, being swapped is like waking up from a dream and one slowly loses details from that dream. That person knows their homeworld isn't a dream. But usually, more than half their memories of their homeworld become fuzzy over time. However, all memories will return when they go back to their original bodies, with the addition of bits and pieces about what their counterpart did in their absence. Therefore, when it comes to long-term swapping, it's a good idea to return to one's original body every once in a while. Even if it's for a brief visit.