The area portals in Innernet teleport the player long distances. One might think that each portal links to another with hard coded coordinates, but this would cause the transition to look very choppy. Instead, each portal just adds an offset to the player's current position. (It also sometimes changes the player's angle to match the output portal's)

By positioning the player partially within a portal and using the bookmarker, the player is teleported to the bookmark and then the area portal's offset is added. Essentially, the offset is added from the bookmark's position, instead of the player's.

This only works for certain portals, however. According to my observations, it only works for pairs of area portals that do not change the player's angle. For instance, here is a rough mockup of one such pair: