The jackpot is triggered by a specific combination of symbols on the reels. When you pull the lever or push the button, the random number generator generates a mixture of symbols. If the given combination matches the jackpot combination, you win big time.
The truth is that there is no trick to slot machines. They provide random results based on set mechanics and it all comes down to luck. With that being said, not all games are the same, so picking the right options is key, and you can still change the size of the bet throughout the session for better results.