I haven't read many TREK novels. If I'm remembering correctly, one novel explained warp drive as hyperspatial jumps. A ship would jump, take location readings, then jump again. One risked getting lost by jumping too far, so long voyages could be tedious. Along came Daystrom's new computers, which could take readings and plot a new course in the blink of an eye. Thus, the jump-jump-jump discontinuity of a starship seems fluid. The pseudo-movement bothers some people new to starflight the way the flicker of fluorescent lighting annoys some people today. At least, that's the way one novel explained it.
Maybe the Valiant
jumped too far and ended up near the Barrier—maybe even bouncing off of it. Or perhaps the Valiant
used a pre-warp FTL engine and traveled backward in time. To the people back home, the Valiant
"disappeared" and covered an impossible distance in "no time," pseudo-instantaneously. Maybe the Valiant
achieved the first engine "implosion," a la "The Naked Time." (Scotty's line in that episode about "regenerating the engines" suggests that they are not physical, but a projected field of some sort.)
"The impossible has happened."