get your friend to pick up something heavy. hopefully not breakable and in an open space. have him throw the heavy thing away from him very hard and have him notice which way he is pushed. action, reaction. i also like the answer that everittmickey gave with the recoil. it is a very good illustration.
the "pushing against" occurs when the rocket's explosive exhaust pushes against the tail of the rocket. the exhaust is the reaction mass. the matter that makes up the exhaust is the reaction mass. the exhaust is a gas and can not act as a compressor against the spacecraft. jumping off the earth is equivalent to you and your feet being the exhaust and the spot on the earth where your feet touches it being the nozzle on the spacecraft. you don't need to push against the air to push against the earth and the earth doesn't need to push against the air to push against you. it's inertia does that. in the nozzle example the gas released pushes against the nozzle. in the the ion thruster example the ions are pushing against the mass driver on the spacecraft. the force is not pushing against the external environment. it is the mass of the particles pushing against the mass driver. the mass driver does not rely on pressure. it relies on a powerful electromagnetic field pushing ions away from the mass of the craft. the fire hose example is even easier. perhaps you have heard the phrase pushing rope. the water coming out of the hose, once it leaves the hose no longer has any ability to push against the hose because it is liquid and can not act as a compression against the hose. it is in the act of leaving the hose that it pushes against the nozzle creating the reaction force.
so, in space, ACTION-REACTION laws are suspended? is that what it is proposed?
nope, not sure i can explain it any more clearly. rockets never did work by pushing against air. the mass of the exhaust pushes against the mass of the rocket. this balls out physics guy is misleading you. try finding every other video you can on 3rd law stuff and watch them. you'll get it. it is nothing to do with pushing against the atmosphere. in space or not in space.