Slot, like any other slot machine in the world, is a game of luck and chance.
What I would try to do is watch your tamagotchi spin around several time so you can see when he smiles and when he faces away.
Then try and press the button at the time when you can remember your tamagotchi smiling. Like for e.g he faces away, then looks angry then smiles try and hit the button just as he is turning angry