50% chance to choose a finale player at random. If not, then it picks a player at random from the whole list.
So if half the players have active finales, they effectively have a 75% chance of appearing, which seems like a lot, until you remember they'd have a 50% chance of appearing anyway.
If you have 4 finale players and 1 replaced player, then the replaced player goes from having a 20% chance of appearing to 10%.