Modern slot machines use random number generators (RNGs) to determine the outcome of each spin, so the result is completely determined by chance. The RNGs are constantly generating new sequences of numbers, even when the machine is not being played, so the timing of when you stop the reels has no effect on the outcome.
Stopping the reels on a slot machine does not affect the outcome of the game. Modern slot machines use a random number generator (RNG) to determine the outcome of each spin.