From the OC-Assist-Dogs mailing list:
Do jackpots convey any additional information? That's what one list member of the OC-Assist-Dogs mailing list wanted to know. A jackpot is an extra special reward meant to tell the dog, "That was outstanding!!" It's usually an extra big treat or several smaller treats delivered one after another.
The response on the list was overwhelmingly negative. Jackpots make the giver feel good, but they interrupt the flow of training and focus the dog on the food, rather than the task. The list recommended a regular treat coupled with effusive praise rather than a sudden change in the amount or type of treat. Overall, it's clarity of criteria and a consistently high rate of reinforcement that leads to a solid behavior.