Let's bring out the old hobbyhorse...
We now have new technobabble to go by, from ENT. Doug Drexler's ideas on what the blue dome thing did in NX-01 might surprisingly well fit previous ideas on what it did in NCC-1701-refit. Drexler's dome adjusts the main warp field, essentially "deflects" it as needed. Sternbach and Okuda in turn suggest that suitably adjusted warp fields reduce inertial mass, making impulse propulsion possible. So an "impulse deflection crystal" could deflect the warp field in order to create inertial mass reduction and allow for impulse propulsion, in NX-01 and NCC-1701 and NCC-1701-B alike.
Sternbach and Okuda also say that a doohickey built into the impulse engines themselves takes care of the inertia trickery in NCC-1701-C, which (perhaps not so coincidentally) lacks the blue dome. NCC-1701 before the refit might also have had such a doohickey, as might ships preceding NX-01. But whenever warp fields became more powerful, or ships more massive, the externally visible dome would have to be brought into play again, until technology evolved and internal doohickeys could take over once more.
I'm liking this one. So one of the functions is essentially to facilitate the reverse impulse manueuver by refocusing the impulse field?
I'll just toss my own 2cents in here... explain why I don't like this... and then let it go.
"Impulse" is a real, meaningful term... much as "force" is a real term, and "mass" is a real term, and "velocity" is a real term, and "addition and subtraction" are real terms. It's so fundamental that attempts to say that it means something else are always going to to be intensely objectionable to me.
That most folks who haven't taken "Dynamics" classes (dealing with the physics of objects in motion) may not know this doesn't change that fact.
It's this reason that makes me cringe every time someone comes up with an explanation for what "impulse" means that has no relationship to what an impulse is. It's not really any different than trying to redefine the term "mass."
If a system called "impulse" doesn't have anything to do with what an impulse is... that's just wrong.
The original coiners of the term knew what it meant, and that's why they coined the term. Some of the writers of the show, and many (I'd go so far as to say "most") of the effects guys, from TOS onwards, didn't necessarily understand their physics, and so things got a little confused... and that's why we're where we are now.
Look... the easiest way to slow down is to simply TURN AROUND. The Enterprise isn't an airplane... despite the fact that it's always shown "facing forwards" there's no real reason not to simply turn around and apply "reverse thrust" in that fashion.
If only just one effects shot had showed this happening, this argument would never come up, either.
That said... we've been given soooo much nonsensical "impulse drive" behavior on-screen, that SOMETHING has to be done. And the idea of a "subspace-assisted impulse" drive has been a favorite of mine for a couple of years now. So TIMO's explanation works... but the idea of it "redirecting the 'impulse field'" doesn't.
There is not an "Impulse field." Rather, with this explanation, there is a subspace field which doesn't do ANYTHING on its own to propel the ship. It just makes it easier for the "rockets"... the impulse thrust system... to do their job, by reducing the "effective mass" that they have to accelerate.