So actually the reel is spinning the values from this array, and when space is pressed it stops right there? 
Yes, that's right. In each iteration the non set positions are updated. Then it checks if space was pressed. In that case sets one of the results.
Also the slot machine is updated 50/60 times per second. So, for a human, I think it is matter of luck.
No, it´s not! You just had to watch those symbols very carefully and hit the spacebar at the exact moment. Three grapes and cherries were really easy to get, I think Konami logos weren´t hard either. Three Penguins may have been a bit harder, but definitely possible.
I remember one pal often got 3 Penguins quite easily.
<--See here!
I was watching the symbol I wanted to get. If I wanted three lemons, I watched carefully for them. I think the exact moment was to press spacebar when wanted symbols were on the winning line. It was that easy! Start practising!
As a kid I used to listen to the music. You needed to make the "x" unit of time same as the beat of the music!
Another way that increased my luck was to press the space time at the same at according to the looping music. This certainly increased my odds! 3x lemons was very easy and others quite common to get, too (but I've always thought 3 x sculls is way more rare than 3 x penguins!).
I also remember trying to see the symbol I wanted to get and press one cycle after it, since it did seem the symbols were cycling (I don't think you can have fast enought reflexes to press the space as you see the symbol). But the music based technique worked better for me, and made it easy enough to win fish (and later I got good enough in the actual game I really didn't need the slot machine at all)
