HA! There is no such thing. It's an option the dealer adds on at Utopia Planetia but it doesn't really do anything, like undercoat protection.
Though seriously, I have seen it a couple of times, for instance, in the new movie. But doesn't it seem like 9 tims out of 10 it fails?
Also: Pretty funny they say to eject the warp core in the new movie, and ten things fly out of the ship. Hopefully no one was in the sonic shower they accidentally jettisoned. And: didn't they use the same thing to stop an artificial singularity in the movie Insurrection as well? Lazy writers.
It wasn't a singularity in Insurrection, it was a subspace tear caused by an isolytic burst.
Also, the relauch movie was supposed to be a new design of many things, such as several smaller warp cores, homage to the horizonal warp cores of the Original series era.
If I recall, Voyager was the only ship with a fully working ejection system. They were able to eject the core everytime they needed to.