I think you're right. You get acceleration in one direction by accelerating and ejecting mass in the opposite direction. The degree to which it's efficient depends on how much of your energy source was converted into kinetic energy of the ejected mass, and how directional that ejection mass was. The relative velocity ("stopped" versus moving relative to some external frame of reference) only determines how much propulsion you're getting with respect to that frame of reference, nothing to do with efficiency at all.

On the other hand, this doesn't make blowing stuff up efficient. An explosion, unless it's lensed properly, will push mass equally in all directions.