Some creatures can naturally move, at will, from one version of reality to another simply by walking, flying, swimming, sailing, running (or whatever). There are even a handful of artifacts will allow people to travel this way, but their rarity and immense value makes them mythical to all but a very few. It works, rather like this: from where one stands, each step forward is the entrance to another reality ‘next door’, another of an infinite number of variations in the multiverse.
So trans-reality movement, sometimes called ‘Hell Walking’, simply allows the being in question to travel to an alternate reality other than the one he is actually in, solely by application of will and magikal power! As he concentrates upon the reality that he wants to be in, the land around him subtly and ever so slowly ‘fades’ from one reality to the next, each change minute but closer to the reality he is traveling to and less like the one he is traveling from. Each change actually puts the traveler into another reality, even if only briefly, and each such reality will have its own ‘laws’ and cosmology. The distance physically covered (and time required to do so) during this type of movement will be exactly the same regardless of the intervening other realities.
What this boils down to is that the longer you travel, the more realities other than your own you must pass through. Every reality you travel away from your own requires the ever-growing expenditure of power, in prodigious amounts. Realities widely different from your own that you must transit are even more draining, multiplying the difficulty by two, three, four or even more times based on how much different; though the vagaries of the endless change of perception is too lengthy to include here.