The marine jet has a water intake in the bottom of the hull that pulls water from under the boat. The engine drives an impeller that forces water out the stern pushing the boat forward. The impeller is like a prop on a boat. It adds mass flow and velocity to the water creating thrust. The balance between mass flow and velocity determines the optimum boat speed for peak efficiency.
Water flow exiting the nozzle is directed port and starboard via a deflector to control turns.
A gate closes over the jet stream to provide neutral and reverse thrust flow. These positions may be modulated to further enhance controllability. You do not have to reverse the engine to reverse the boat. Simply by lowering the reverse gate you can reverse the direction or stop the boat, even at full throttle.