Two important techniques that many spirit-workers and shamans learn to master are pathwalking and horsing (to use terms common these days in various spirit-worker communities). Pathwalking, as Raven Kaldera defines it, is "the technique of walking in two worlds at once. In order to do it, you 'pull in' the other world and superimpose it on this one. Your physical body walks in your home world, while your astral body moves through the second world." Horsing covers a wide spectrum of ways in which you can let a god or spirit enter and control your body, sometimes used to describe states ranging anywhere from "aspecting" to being fully "possessed." While these experiences are usually discussed separately and without reference to each other,  I find many similarities between the two, to the point where I would posit that they are describing the same basic process, only applied in one instance to entire worlds/realities, and on the other hand to individual entities (humans, gods, spirits). This is most clear when discussing the deliberate pursuit of such experiences by spiritual specialists such as shamans, shamanists and spirit-workers, including the induction of altered states of consciousness (ASC), rather than the occasional involuntary aspecting or natural parting of the veil. 

A spirit-worker who pathwalks is essentially opening a door to allow a world Over There (i.e., in the otherworlds, a place on the astral, a spiritual realm, or however you classify it) to "ride" this world (our physical world, Midgard), much in the way that a god or spirit rides a human "horse" during possession. 

Some places in our world are always open a crack, more easily susceptible to this dual perception because they are already numinous. Similarly, some people are also naturally open and are more easily ridden by spirits. Sometimes this is involuntary, a form of insanity. For spirit-workers and shamans, it is simply part of the job description. A spirit-worker (or at times, a devotee) can deliberately open themselves further, to the point of full possession, via the use of ritual, prayer and various methods for attaining ASC (along with, of course, the cooperation of the gods involved). There are also times they might be just naturally more open - during other times of prayer and ritual, certain times of the year, month, or even day, times of communion with their gods, etc. Likewise, a physical place can be deliberately "opened" by the spirit-worker via ritual in order for the otherworlds to ride in and be superimposed on this world, but it may also be naturally more susceptible to this at certain times (e.g., seasonal variations and times of power such as high holy days) and under other circumstances (e.g., as a result of events taking place on the land, or the activities of the land spirits). 

Some places always lead to the same Otherworld (in folklore, this might be spoken of as there being a gate there to Fairyland, or the Underworld, or a very specific place like Svartalfheim). Other places can shift where they lead to. Just like some "horses" are only available to be ridden by a particular god (usually their patron), but others are available for many. In both cases, though, there needs to be some connection or similarity between the places or beings. It is hard to pathwalk in a watery otherworld if you're physically in the desert. It may also be more difficult to horse a god of the opposite gender, or one with radically different disposition, or one that you've never met before. 

Other people might come across a world riding this one, a place where the veil is thin, without any doing on their part. But to intentionally open the gates for that to happen, on a regular basis, that is the work of a shaman or spirit-worker, just like they invite the spirits into themselves rather than being overcome without warning. One world rides another just like one being rides another, and the necessity for skill and experience in both cases is equally important. Both to avoid disaster and to ensure that the experience is meaningful and accomplishes what it needs to.