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.
Check out how often it pays out.\n\n However, not all machines are created equal; some pay more often than others and some rarely do so at all. A machine with many rewards is a highly volatile one because a player has to wait longer between wins but when they come they tend to be bigger.