DRAFT - SUBJECT TO CHANGE
The Detent Drive is the means of effectively faster-than-light travel used in the FE universe.
How it works is not known either by humans or by galactic civilization in general, and its behavior is understood by them through analogical models and empirical results. Traditionally, the Detent Drives of ships were operated by the Outer Priesthood, though since the beginning of the War of Religion, they have increasingly fallen under the control of others, including, recently, humans.
The key analogy to understand how the Detent Drive works is to imagine familiar four-dimensional spacetime to be analogous to a notionally 2D flat piece of cloth which can be bunched, layered, and folded with recourse to the third spatial dimension. 3D space is found to be correspondingly "folded and layered" in higher spatial dimensions, and thus points which are extremely distant from one another in 3D space may found to be quite close when the higher-dimensional structure of space is considered. It is also found that things are normally attracted to the lower dimensions by a fifth fundamental force, which is why for example matter with higher-dimensional velocities is not seen to appear and disappear as it passes through areas of three-dimensional space.
(Bryce: Why is it not possible to oscillate back and forth through 3D space, if the fifth force works like gravity or electromagnetism?
Reese: Well, the 4D space would appear to be significantly nonuniform on a large scale, if there's one particularly 3D layer in it (our conventional 3D space) to which things are attracted. Perhaps there are different forms of matter, which we haven't noticed because they're differentiated by a property which all matter in our 3D space possesses equally, and matter with different values of this property are attracted to different layers in the 4D space? Or perhaps it's more of a gradient thing, and our 3D space is "floating" on top of layers of 3D space filled with "denser" matter? Hm. Anyway, I was aiming for some way to provide a damping force, such that such oscillations could occur but, unless actively driven, would relatively cease with the matter either entirely in or entirely out of our 3D space. Hopefully this bit of brainstorming's useful.)
Bryce: I think a damping is what's needed, or maybe there are two forces with different strengths?