Pops advance Bar Ghost or War Fort when no modes are active. You can decrease the number of hits it takes in the Game Menu.
All modes have 1 "quick/easy" restart per player per game. If you start Bar Ghost, fail, restart, fail again Pops will then advance War Fort so you can try something else.
If you complete War or Bar, pops advance the other mode.
If both are complete, or if you're in a mode, the pops advance the Ghost Battle jackpot and every 10 pops collects an EVP. In the next code revision, you can set X number of EVP's collects an Extra Ball.
To manually select what mode the pops start in (rather than random) hold left flipper for Bar Ghost, right flipper for War Fort, then press START to begin game. This only works for Player 1, other players will remain random.
In hindsight, the pop logic is WAY too convoluted - it's confusing for players and was super tough to program.
I'd suggest trying to complete the pop bumper modes (Bar and Fort) early, so the pops spend more time for the rest of the game advancing Jackpot Value.
EDIT: 12 more on the way!