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.
It's all a matter of chance. There are no blackjack- or video poker-like strategies that can cut into the house's mathematical edge. Odds on slot machines are unchanging. There's nothing you can do legally that will change the outcome, though scam artists sometimes have succeeded until caught, arrested and imprisoned.