The momentum of the mass expelled in the opposite direction ... the rocket-engine exhaust, or the ionized matter expelled from an ion drive.
THAT's why every propulsion engine has outlet nozzles designed with super-high-intensity math, to achieve the highest possible velocity for the mass that gets shot out the back ... so that it will carry the maximum possible momentum.
I think your talking about refraction and if so the first medium it existed in before entering the second usually has the higher speed especially it the first medium is less denser than the second
