This is incorrect, I think.
On any ball, once any switch is hit, the ball save starts to flash. However, if you do not it any optos, rollover switches, or the ram, you have I believe 3 more switch hits until the ball saver actually starts to time out.
Now if you ever start ball 2,3,4, or 5 and the ball saver light is flashing with he ball in the shooter lane, that means you have just started your extra ball you've earned on the previous ball, but all the rules of my first paragraph still apply.
Also, if after locking a ball you manual plunge and avoid any roll overs, that same 4 switch rule in in effect if you happen to brick a couple shots and drain down the middle.
Once I learned all this, and that the theme song changes once the playfield was validated, I hardly ever pay attention to the ball saver light anymore.