All possible resulting hands and pays for a hold of just the ace of hearts must be calculated. The same must then be done for a save of just the 3 of spades, the 4 of hearts, the 5 of clubs, and the king of diamonds. Then the same must be done for each possible hold of two cards. Then the same is done for holds of three cards. The same is done for holds of four cards. Finally the return for a hold of all five cards is calculated. The returns are then compared in order to select the best possible hold (in terms of money returned). The results for each of the over two and a half million possible hands are summarized in order to develop the strategy.

Once I hit two 4 of a Kinds back to back.  The probability of hitting it once is 0.24%.  The chances of hitting it back to back are 0.24%2 = 0.0006%, or 1 in 179,000.  That's four times less likely than hitting a royal flush.  And I would have preferred the royal!  (By the way, purists will note that after you've hit a 4 of a Kind, the chances of hitting another four of a kind is just 0.24%, not 0.0006%, but I'm considering the probability of getting two back-to-back before I get the first one.)
A kicker is simply a card from a certain set of cards. For Double Double Bonus, four aces pays 160 for 1. However, four aces with a kicker of a 2, 3 or 4 pays a whopping 400 for 1. This is two and one half times what is paid for four aces without a kicker. Getting four aces with a kicker pays half as much as a royal and it occurs two and a half times as often. Four 2’s, 3’s, or 4’s pay 80 for 1. Four 2’s, 3’s, or 4’s with a kicker of an ace, 2, 3, or 4 pays double that or 160 for 1.
So it really pays to hunt out the good machines!  But it's actually pretty easy, because the readers of a website called VPFree2 scout out the best machines and post their locations on the site.  Using that site you can see that you can there are 9/6 Jacks or Better machines on the Strip at Cosmopolitan, New York New York, Riviera, Treasure Island, and others.  And in downtown and off-strip casinos, the good games are more common, and available at lower denominations.
Well, the charts on our site are on roughly accurate to within 0.1% of perfect play. So, if you find that game that returns 100.5%, you would be actually earning about 100.4% with our charts - not too bad. Not many charts get much closer than 0.1% because they would have to be extremely long and complicated to explain the subtle differences in rare hands.