Ask The Wizard #204
When the meter crosses the predestined hit point, everybody with a slot card in and playing will win $50 in fee play. To be considered “playing” the player must have his player card inserted, and have made a bet within the last ten seconds. Then, somehow, a machine is chosen at random from all those being actively played to win the Jumbo Jackpot. It does not appear that the bet amount matters, so all qualifying machines have the same probability of being chosen. As long as you can actively play multiple machines, that would muliply your chances of the jackpot by number of machines played, and you would qualify for the free play on all of them.
I would like to thank Bob Dancer for his help with this question.
Note: This answer has been updated in June, 2008, after a rule change to the Jumbo Jackpot.