Named for a team of Warp-physicists the Peatreu-T'proul Effect describes why warp-capable ships are almost always oriented within the same plane in space.
From the earliest days of warp travel it was noticed that warp capable ships, especially those with multiple nacelles have small but noticeable gains in efficiency if they travel within the Galactic plane or system plane if close to a star system. While many theories were floated about it wasn't until a team of physicists at The Oxford Annex campus on Vulcan were able to finally track down and measure the sub-spatial drag that gravity causes as an object spins in space. Just as a black hole literally drags space with it as it rotates all objects pull a similar drag in subspace. The higher off-plane you are the amount of distortion increases linearly. When travelling at an angle to the plane of the system the slight differences in distortion translate to lower warp efficiency and increased wear on the drives.
In intergalactic space the effect is almost unnoticeable due to the comparative sizes of ships and the galaxy as a whole. The effect becomes more pronounced near stars or other large bodies. The boundary region between a star system and intergalactic space where a ship should realign to the local plane is is the Peatreu-T'proul boundry. Often called the doorstep, it takes tight timing to change alignments without causing a bump as the ship passes through the interface. On almost all mapped systems this is handled by computer, but out of date measurements or entering an unknown system often causes the ship to bump or trip. Usually to the embarrassment of the helmsman. Tripping the ship while harmless is often a sign of a rookie officer and almost all have done it at some point.