As a mechanical design engineer for over 30 years, I'm really surprised how many people on this forum don't realize how a simple inclined plane works.
The picture above (just like the OP's revolver) shows a perfectly machined locking angle and plunger ball.
The force from the plunger spring pushing the ball against the OUTSIDE angle (right side as shown) provides an INWARD force pushing the crane closed, which is the WHOLE PURPOSE of the forward lock.
If the ball was centered in the notch, there would be ZERO force pushing the crane closed.
If the ball was slightly to the LEFT of center in the notch, the ball would push the crane OPEN until the ball centered in the notch.
Smith and Wessons note that the notch and ball are perfectly machined is correct.