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?