Can't say exactly how it is explained in video game mechanics, but Wobbly Goblin sounds about right. In SC, when you throttle down, you slow down, even in a space (I think this is solely for video game fun, but I am no scientist).
If your throttle is up and then you decouple, you continue momentum in the direction you were going before you decoupled. Then you can use your maneuvering thrusters to change which direction you are facing, while still traveling in the direction and speed you were. When you recouple, your main engines push you in your new alignment.
On slower moving ships, you can do this to turn drastically faster. While shooting, this turns your whole ship into a gimballed weapon platform.
All the ships seem to behave differently decoupled, I suspect due to the maneuvering thruster layouts. Default key is C. Try it out and see what you think.
By the way, this also will let you make very fast turns in cruise mode. It works well when jumping in and out of fights. But when you recouple in cruise, the blackout and redout danger is very real. Rolling can compensate to some extent.