Why does a stream change trajectory?

Bboy Physics

Okay bear with me here because this might get disgusting. But I was watching Henry's video on how astronauts went to the bathroom in space, and basically they did it in a bag, and shot the bag outside into outerspace. And then he mentioned that the trajectory would change(but extremely small yet significant enough to tell Hudson Control when you're flying to the moon). I realize that streams change trajectory(comedy in firing an extremely high powered hose), but what is this force called?

fzero

You could call it the reaction (force). It is a consequence of Newton's third law, or, in a more modern perspective, of conservation of momentum.