Subspace Drive

Subspace Drive (known also as Ghost Drive, Phaseshift Drive and Subspace/Ghost/Phaseshift Engine) is a rather high-tech addon to the Hyperspace Drive, which is considered a part of the so called 4,5 gen. Hyperspace Drive together with Void Drive. It isn't something completely unique, since it doesn't allow the ship to enter Hyperspace, making actual journeys between star systems by using Subspace Drive a rather unlikely occurence.
Subspace is believed to not be any 'dimension' like a Hyperspace or Void. Instead, it is a state. Subspace Drive works by partially shifting a ship into a rather weird form of a Ghost Ship (hence it's alternative name, Ghost Drive), making it exist somewhere between Hyperspace and Realspace.
This allows the ship actually achieve FTL speed in Realspace. The best way to describe it is saying that while the ship physically still exists (mostly) in Reality, it is pushed deep enough into Hyperspace for its rules to take precedence (or just be a thing), thus making the ship travel faster than it should. Because while it still exists in Realspace, the actual movement occurs at least partially in Hyperspace.
To say the Subspace Drive is... inconvenient for the crews would be an understatement. Trying to travel too far will probably break firstly the crew (in both physical and mental meaning), and then the ship itself. Many subspace jumps actually makes the ship better attuned to the halfway state, limiting the dangers of getting broken into pieces, but the effects on the crew mostly remain. And even then, consecutive jumps aren't safe even if short.
Ships travelling in Subspace can be detected by sensors, the only problem being the fact that all of their input is pretty much out of sync. Even the best sensors can detect things at the speed of light, making ships able to move faster than that a thing rather troublesome to keep track off. The faster it goes, the more behind the sensors are. The ships cannot be damaged by any normal mean, because during the jump their matter is also partially phased out and only a relatively small part of matter comes in contact with them (making anything short of a planet unlikely to stop the phasing ship.
Because of its dangers, requirement of high technology and relatively low usage outside of combat, Subspace Drives are limited to military ships, and only from the more advanced countries.