The "ball saver" you reference to is a lower level stage in the state machine of the pinball machine logic.
New ball (/new player) -> shooter lane -> ball in play -> drain.
With draining during shooter lane state reserving ball and staying in shooter lane state.
On Williams games (traditionally): Shooter lane state, the score is blinking. Ball in play state, the score is slowly rolling.
In order to determain the advancement to ball in play, the game logic needs to see the ball in action on the playfield. This is traditionally a switch hit out of trough/shooter lane. And was later in the 90's on Williams change to three different switches (same for Stern/JJP now a days). But is highly individual per game. On some games some switches are still a one hit one. On some games a ball saver can trigger the advancement. It is just a mess.
All this to make sure that a player is not drained prematurely. Because if this happens, the player will walk away never playing the game again. Possibly vandalising the game or, even worse, complaining to the staff demanding a refund.
With the introduction of ball saver (circa T2), the path is...
New ball (/new player) -> shooter lane -> ball in play -> ball saver -> drain.
With draining during ball saver state reserving the ball and, where available, auto plunging it. And staying in ball saver state with the timer not reset.
So, after this long explaination. A ball saver is a new feature to make on game lacking it. It is timed, ball in play logic is not. And, should be backed up by lights (shoot again), DMD, sound effects and speech. With DMD being hard (for something that looks half decent) and sound not possible.