My understanding is that it is to do with pipes, bottlenecks and timing,
Which to me, means you can have simultaneous processes occurring (cores), but then they share output and input pathways, and that is where timing comes into it. First come first served is where I imagine the racing occurs.
I am however far from being an expert, and so could be completely wrong in what my mind has pieced together, from things I have read and what seems logical. It has also been some time between readings ... usually only when considering my next PC.
I have forgotten for instance, where the Southbridge fits into all that ... just remember it improves things in that regard ... alternate pathways or something.
My advice, is don't get old ... you forget or misremember things ..... oops too late.