Originally Posted by JayUtah

Is that a decent explanation?

It's a decent explanation of pressure thrust, which describes the thrust component derived from the net static pressure of the exhaust gas as it leaves the thrust chamber. This can account for a substantial percentage of the total thrust an engine produces in a vacuum.

But it doesn't capture the mechanism of momentum thrust.

In the ball-and-hole model, the direction in which gases exit the hole wouldn't matter. They could go straight out the back, or they could fan out as intuition suggests they would naturally do. But in conservation of momentum, we find that direction does matter. Momentum is a vector quantity because velocity is a vector quantity. The more particles that depart directly along the axis of motion, the greater the momentum reaction in the rocket. A rocket exhaust that departs in a neat column causes the strongest conservation-of-momentum reaction in the rocket (which is constrained by its solid nature to make all its particles go the same direction).

Thence nozzles. By fitting the hole in the ball with a nozzle that directs fluid flow into a column, you ensure that the departing particles stay in a column as much as possible, keeping their velocity vectors reasonably aligned and setting up the cumulative momentum better. Under equivalent mass-flow conditions, the nozzled ball will perform better. That makes the unbalanced-reaction model incomplete.

Keep in mind also that there are engine designs that throw mass by means other than generating fluid pressure. The goal in any Newtonian engine is to throw mass very fast. A common way to do that is create a large amount of pressure in a fluid and limit how it can escape; in turn accomplished by heating it through an exothermic chemical reaction and allowing it to expand, or by catalyzing a decomposition reaction that results in greater volume. But ion engines throw mass by accelerating it through magnetic fields. There is no unbalanced reaction -- no "ball with a hole" -- just pure momentum.