From what I've read, it's best to think of the blade mathematically as an ellipse or ellipsoid with an exceptionally small conjugate diameter, rather than as a straight line. Instead of creating a line of energy that stops at an arbitrary point, it's like creating a BALL of energy that is exceptionally distended in a specific way.
In simpler terms, think of somehow focusing a laser to travel in a very tight circle rather than a straight line, so the light loops around in the shape of a sword blade. Of course, the problem is the "somehow". That's what makes this science fiction. We don't know how to do that right now, but perhaps you can just imagine someone discovering a principle that makes it possible.
The other way I've heard this explained is as if you are accelerating something like ion particles to near-light speed. Additionally, there is a something like a strong magnet in the handle, with the opposite charge as the ions. So the ions shoot out the emitter, but are attracted to the magnet such that they are decelerated enough to stop about a meter from the handle and drawn back the other way until they are recovered.
I say "something like" ions, because if it were ions and magnets the device would interfere with and be itself interfered with by too many normal materials. But, again, science fiction. You can suspend disbelief by understanding it as a particle type and counter-force we just haven't discovered yet.