It only really has any effect two times during gameplay.
Once during the spiral mode, where you have to pass all three locks (or more accurately the optos for each lock) to achieve spiral breakthrough.
Second as you mentioned is during the multiball sequence. The machine launches all three balls for multiball and locks one on each magnet before releasing them simultaneously. The catch is that for it to work the pinball machine must know that the powerball is in the gumball machine - in other words during the same game you must have locked the gumball in the gumball machine first and then achieve multiball. Not always the easiest to do on a standard 3 ball game.
It is good to see how the machine software was originally intended to handle the extra magnet before the decision was made to remove it at BW, but it is a lot of effort and unless you were planning to do a full shopout/restore (as I did) involving a total playfield strip (on the topside at least) probably wouldn't do it again.
Here is a vid of me forcing a multiball sequence by hand as described above. Sorry about the vid quality.
