It is not "faster than light", but if you had (and its a big if) the Portal Gun (https://theportalwiki.com/wiki/Handheld_Portal_Device) you could have a sub-light drive that worked like this.
Create the two portals facing one another across a room in your spaceship. Put a mag-lev track between them and start speeding up a train. As you speed the train up going in the -x direction the reaction force accelerates the rest of the spaceship in the +x direction. At this point you could have just fired the train out the back like a jet plume.
But, because of the portals, you don't loose the train. It is still their going through your ship again and again. (repeating the same length of track). When you want to stop all the kinetic energy and momentum you put in is still their waiting to be harvested back out. (Minus losses, which would be significant but could be made lower if your train went through a vacuum).
This is sub-light. You would have to handwave something extra to make the concept FTL. (You also have to explain why they don't just use portals to get around, maybe the range is limited).