This may be a silly question, but I don't know how to approach an answer myself and I can't seem to find the right search terms to input.
A simple rocket might be a compressed air bottle, where a reaction does not take place and the exhaust gas is the same form as the exhaust itself. Thrust is produced because the air wishes to neutralize its pressure with the ambient, but this could be supplemented by a gas container that dramatically increases the pressure on the exhaust gas (or whatever form of matter), thus increasing exhaust velocity. There are many ways to do this, like increasing the temperature of the exhaust gas while maintaining volume, but I'm thinking of a complicated mechanical vessel or maybe a "sac" that can forcefully constrict and shrink.
Is there any case for hypothetical rocket engines that "squeeze" material in this way, rather than combustion or nuclear engines? Are there any sources I can read that might discuss it?